sqlalchemy / lib / sqlalchemy / orm / mapper.py

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# orm/mapper.py
# Copyright (C) 2005-2013 the SQLAlchemy authors and contributors <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""Logic to map Python classes to and from selectables.

Defines the :class:`~sqlalchemy.orm.mapper.Mapper` class, the central
configurational unit which associates a class with a database table.

This is a semi-private module; the main configurational API of the ORM is
available in :class:`~sqlalchemy.orm.`.

"""
from __future__ import absolute_import

import types
import weakref
from itertools import chain
from collections import deque

from .. import sql, util, log, exc as sa_exc, event, schema, inspection
from ..sql import expression, visitors, operators, util as sql_util
from . import instrumentation, attributes, exc as orm_exc, loading
from . import properties
from .interfaces import MapperProperty, _InspectionAttr, _MappedAttribute

from .base import _class_to_mapper, _state_mapper, class_mapper, \
        state_str, _INSTRUMENTOR
from .path_registry import PathRegistry

import sys


_mapper_registry = weakref.WeakKeyDictionary()
_already_compiling = False

_memoized_configured_property = util.group_expirable_memoized_property()


# a constant returned by _get_attr_by_column to indicate
# this mapper is not handling an attribute for a particular
# column
NO_ATTRIBUTE = util.symbol('NO_ATTRIBUTE')

# lock used to synchronize the "mapper configure" step
_CONFIGURE_MUTEX = util.threading.RLock()


@inspection._self_inspects
@log.class_logger
class Mapper(_InspectionAttr):
    """Define the correlation of class attributes to database table
    columns.

    The :class:`.Mapper` object is instantiated using the
    :func:`~sqlalchemy.orm.mapper` function.    For information
    about instantiating new :class:`.Mapper` objects, see
    that function's documentation.


    When :func:`.mapper` is used
    explicitly to link a user defined class with table
    metadata, this is referred to as *classical mapping*.
    Modern SQLAlchemy usage tends to favor the
    :mod:`sqlalchemy.ext.declarative` extension for class
    configuration, which
    makes usage of :func:`.mapper` behind the scenes.

    Given a particular class known to be mapped by the ORM,
    the :class:`.Mapper` which maintains it can be acquired
    using the :func:`.inspect` function::

        from sqlalchemy import inspect

        mapper = inspect(MyClass)

    A class which was mapped by the :mod:`sqlalchemy.ext.declarative`
    extension will also have its mapper available via the ``__mapper__``
    attribute.


    """

    _new_mappers = False

    def __init__(self,
                 class_,
                 local_table=None,
                 properties=None,
                 primary_key=None,
                 non_primary=False,
                 inherits=None,
                 inherit_condition=None,
                 inherit_foreign_keys=None,
                 extension=None,
                 order_by=False,
                 always_refresh=False,
                 version_id_col=None,
                 version_id_generator=None,
                 polymorphic_on=None,
                 _polymorphic_map=None,
                 polymorphic_identity=None,
                 concrete=False,
                 with_polymorphic=None,
                 allow_partial_pks=True,
                 batch=True,
                 column_prefix=None,
                 include_properties=None,
                 exclude_properties=None,
                 passive_updates=True,
                 eager_defaults=False,
                 legacy_is_orphan=False,
                 _compiled_cache_size=100,
                 ):
        """Return a new :class:`~.Mapper` object.

        This function is typically used behind the scenes
        via the Declarative extension.   When using Declarative,
        many of the usual :func:`.mapper` arguments are handled
        by the Declarative extension itself, including ``class_``,
        ``local_table``, ``properties``, and  ``inherits``.
        Other options are passed to :func:`.mapper` using
        the ``__mapper_args__`` class variable::

           class MyClass(Base):
               __tablename__ = 'my_table'
               id = Column(Integer, primary_key=True)
               type = Column(String(50))
               alt = Column("some_alt", Integer)

               __mapper_args__ = {
                   'polymorphic_on' : type
               }


        Explicit use of :func:`.mapper`
        is often referred to as *classical mapping*.  The above
        declarative example is equivalent in classical form to::

            my_table = Table("my_table", metadata,
                Column('id', Integer, primary_key=True),
                Column('type', String(50)),
                Column("some_alt", Integer)
            )

            class MyClass(object):
                pass

            mapper(MyClass, my_table,
                polymorphic_on=my_table.c.type,
                properties={
                    'alt':my_table.c.some_alt
                })

        See also:

        :ref:`classical_mapping` - discussion of direct usage of
        :func:`.mapper`

        :param class\_: The class to be mapped.  When using Declarative,
          this argument is automatically passed as the declared class
          itself.

        :param local_table: The :class:`.Table` or other selectable
           to which the class is mapped.  May be ``None`` if
           this mapper inherits from another mapper using single-table
           inheritance.   When using Declarative, this argument is
           automatically passed by the extension, based on what
           is configured via the ``__table__`` argument or via the
           :class:`.Table` produced as a result of the ``__tablename__``
           and :class:`.Column` arguments present.

        :param always_refresh: If True, all query operations for this mapped
           class will overwrite all data within object instances that already
           exist within the session, erasing any in-memory changes with
           whatever information was loaded from the database. Usage of this
           flag is highly discouraged; as an alternative, see the method
           :meth:`.Query.populate_existing`.

        :param allow_partial_pks: Defaults to True.  Indicates that a
           composite primary key with some NULL values should be considered as
           possibly existing within the database. This affects whether a
           mapper will assign an incoming row to an existing identity, as well
           as if :meth:`.Session.merge` will check the database first for a
           particular primary key value. A "partial primary key" can occur if
           one has mapped to an OUTER JOIN, for example.

        :param batch: Defaults to ``True``, indicating that save operations
           of multiple entities can be batched together for efficiency.
           Setting to False indicates
           that an instance will be fully saved before saving the next
           instance.  This is used in the extremely rare case that a
           :class:`.MapperEvents` listener requires being called
           in between individual row persistence operations.

        :param column_prefix: A string which will be prepended
           to the mapped attribute name when :class:`.Column`
           objects are automatically assigned as attributes to the
           mapped class.  Does not affect explicitly specified
           column-based properties.

           See the section :ref:`column_prefix` for an example.

        :param concrete: If True, indicates this mapper should use concrete
           table inheritance with its parent mapper.

           See the section :ref:`concrete_inheritance` for an example.

        :param eager_defaults: if True, the ORM will immediately fetch the
          value of server-generated default values after an INSERT or UPDATE,
          rather than leaving them as expired to be fetched on next access.
          This can be used for event schemes where the server-generated values
          are needed immediately before the flush completes.   By default,
          this scheme will emit an individual ``SELECT`` statement per row
          inserted or updated, which note can add significant performance
          overhead.  However, if the
          target database supports :term:`RETURNING`, the default values will be
          returned inline with the INSERT or UPDATE statement, which can
          greatly enhance performance for an application that needs frequent
          access to just-generated server defaults.

          .. versionchanged:: 0.9.0 The ``eager_defaults`` option can now
             make use of :term:`RETURNING` for backends which support it.

        :param exclude_properties: A list or set of string column names to
          be excluded from mapping.

          See :ref:`include_exclude_cols` for an example.

        :param extension: A :class:`.MapperExtension` instance or
           list of :class:`.MapperExtension` instances which will be applied
           to all operations by this :class:`.Mapper`.  **Deprecated.**
           Please see :class:`.MapperEvents`.

        :param include_properties: An inclusive list or set of string column
          names to map.

          See :ref:`include_exclude_cols` for an example.

        :param inherits: A mapped class or the corresponding :class:`.Mapper`
          of one indicating a superclass to which this :class:`.Mapper`
          should *inherit* from.   The mapped class here must be a subclass
          of the other mapper's class.   When using Declarative, this argument
          is passed automatically as a result of the natural class
          hierarchy of the declared classes.

          See also:

          :ref:`inheritance_toplevel`

        :param inherit_condition: For joined table inheritance, a SQL
           expression which will
           define how the two tables are joined; defaults to a natural join
           between the two tables.

        :param inherit_foreign_keys: When ``inherit_condition`` is used and the
           columns present are missing a :class:`.ForeignKey` configuration,
           this parameter can be used to specify which columns are "foreign".
           In most cases can be left as ``None``.

        :param legacy_is_orphan: Boolean, defaults to ``False``.
          When ``True``, specifies that "legacy" orphan consideration
          is to be applied to objects mapped by this mapper, which means
          that a pending (that is, not persistent) object is auto-expunged
          from an owning :class:`.Session` only when it is de-associated
          from *all* parents that specify a ``delete-orphan`` cascade towards
          this mapper.  The new default behavior is that the object is auto-expunged
          when it is de-associated with *any* of its parents that specify
          ``delete-orphan`` cascade.  This behavior is more consistent with
          that of a persistent object, and allows behavior to be consistent
          in more scenarios independently of whether or not an orphanable
          object has been flushed yet or not.

          See the change note and example at :ref:`legacy_is_orphan_addition`
          for more detail on this change.

          .. versionadded:: 0.8 - the consideration of a pending object as
            an "orphan" has been modified to more closely match the
            behavior as that of persistent objects, which is that the object
            is expunged from the :class:`.Session` as soon as it is
            de-associated from any of its orphan-enabled parents.  Previously,
            the pending object would be expunged only if de-associated
            from all of its orphan-enabled parents. The new flag ``legacy_is_orphan``
            is added to :func:`.orm.mapper` which re-establishes the
            legacy behavior.

        :param non_primary: Specify that this :class:`.Mapper` is in addition
          to the "primary" mapper, that is, the one used for persistence.
          The :class:`.Mapper` created here may be used for ad-hoc
          mapping of the class to an alternate selectable, for loading
          only.

          The ``non_primary`` feature is rarely needed with modern
          usage.

        :param order_by: A single :class:`.Column` or list of :class:`.Column`
           objects for which selection operations should use as the default
           ordering for entities.  By default mappers have no pre-defined
           ordering.

        :param passive_updates: Indicates UPDATE behavior of foreign key
           columns when a primary key column changes on a joined-table
           inheritance mapping.   Defaults to ``True``.

           When True, it is assumed that ON UPDATE CASCADE is configured on
           the foreign key in the database, and that the database will handle
           propagation of an UPDATE from a source column to dependent columns
           on joined-table rows.

           When False, it is assumed that the database does not enforce
           referential integrity and will not be issuing its own CASCADE
           operation for an update.  The :class:`.Mapper` here will
           emit an UPDATE statement for the dependent columns during a
           primary key change.

           See also:

           :ref:`passive_updates` - description of a similar feature as
           used with :func:`.relationship`

        :param polymorphic_on: Specifies the column, attribute, or
          SQL expression used to determine the target class for an
          incoming row, when inheriting classes are present.

          This value is commonly a :class:`.Column` object that's
          present in the mapped :class:`.Table`::

            class Employee(Base):
                __tablename__ = 'employee'

                id = Column(Integer, primary_key=True)
                discriminator = Column(String(50))

                __mapper_args__ = {
                    "polymorphic_on":discriminator,
                    "polymorphic_identity":"employee"
                }

          It may also be specified
          as a SQL expression, as in this example where we
          use the :func:`.case` construct to provide a conditional
          approach::

            class Employee(Base):
                __tablename__ = 'employee'

                id = Column(Integer, primary_key=True)
                discriminator = Column(String(50))

                __mapper_args__ = {
                    "polymorphic_on":case([
                        (discriminator == "EN", "engineer"),
                        (discriminator == "MA", "manager"),
                    ], else_="employee"),
                    "polymorphic_identity":"employee"
                }

          It may also refer to any attribute
          configured with :func:`.column_property`, or to the
          string name of one::

                class Employee(Base):
                    __tablename__ = 'employee'

                    id = Column(Integer, primary_key=True)
                    discriminator = Column(String(50))
                    employee_type = column_property(
                        case([
                            (discriminator == "EN", "engineer"),
                            (discriminator == "MA", "manager"),
                        ], else_="employee")
                    )

                    __mapper_args__ = {
                        "polymorphic_on":employee_type,
                        "polymorphic_identity":"employee"
                    }

          .. versionchanged:: 0.7.4
              ``polymorphic_on`` may be specified as a SQL expression,
              or refer to any attribute configured with
              :func:`.column_property`, or to the string name of one.

          When setting ``polymorphic_on`` to reference an
          attribute or expression that's not present in the
          locally mapped :class:`.Table`, yet the value
          of the discriminator should be persisted to the database,
          the value of the
          discriminator is not automatically set on new
          instances; this must be handled by the user,
          either through manual means or via event listeners.
          A typical approach to establishing such a listener
          looks like::

                from sqlalchemy import event
                from sqlalchemy.orm import object_mapper

                @event.listens_for(Employee, "init", propagate=True)
                def set_identity(instance, *arg, **kw):
                    mapper = object_mapper(instance)
                    instance.discriminator = mapper.polymorphic_identity

          Where above, we assign the value of ``polymorphic_identity``
          for the mapped class to the ``discriminator`` attribute,
          thus persisting the value to the ``discriminator`` column
          in the database.

          .. seealso::

            :ref:`inheritance_toplevel`

        :param polymorphic_identity: Specifies the value which
          identifies this particular class as returned by the
          column expression referred to by the ``polymorphic_on``
          setting.  As rows are received, the value corresponding
          to the ``polymorphic_on`` column expression is compared
          to this value, indicating which subclass should
          be used for the newly reconstructed object.

        :param properties: A dictionary mapping the string names of object
           attributes to :class:`.MapperProperty` instances, which define the
           persistence behavior of that attribute.  Note that :class:`.Column`
           objects present in
           the mapped :class:`.Table` are automatically placed into
           ``ColumnProperty`` instances upon mapping, unless overridden.
           When using Declarative, this argument is passed automatically,
           based on all those :class:`.MapperProperty` instances declared
           in the declared class body.

        :param primary_key: A list of :class:`.Column` objects which define the
           primary key to be used against this mapper's selectable unit.
           This is normally simply the primary key of the ``local_table``, but
           can be overridden here.

        :param version_id_col: A :class:`.Column`
           that will be used to keep a running version id of rows
           in the table.  This is used to detect concurrent updates or
           the presence of stale data in a flush.  The methodology is to
           detect if an UPDATE statement does not match the last known
           version id, a
           :class:`~sqlalchemy.orm.exc.StaleDataError` exception is
           thrown.
           By default, the column must be of :class:`.Integer` type,
           unless ``version_id_generator`` specifies an alternative version
           generator.

           .. seealso::

              :ref:`mapper_version_counter` - discussion of version counting
              and rationale.

        :param version_id_generator: Define how new version ids should
          be generated.  Defaults to ``None``, which indicates that
          a simple integer counting scheme be employed.  To provide a custom
          versioning scheme, provide a callable function of the form::

              def generate_version(version):
                  return next_version

          Alternatively, server-side versioning functions such as triggers,
          or programmatic versioning schemes outside of the version id generator
          may be used, by specifying the value ``False``.
          Please see :ref:`server_side_version_counter` for a discussion
          of important points when using this option.

          .. versionadded:: 0.9.0 ``version_id_generator`` supports server-side
             version number generation.

          .. seealso::

             :ref:`custom_version_counter`

             :ref:`server_side_version_counter`


        :param with_polymorphic: A tuple in the form ``(<classes>,
            <selectable>)`` indicating the default style of "polymorphic"
            loading, that is, which tables are queried at once. <classes> is
            any single or list of mappers and/or classes indicating the
            inherited classes that should be loaded at once. The special value
            ``'*'`` may be used to indicate all descending classes should be
            loaded immediately. The second tuple argument <selectable>
            indicates a selectable that will be used to query for multiple
            classes.

            .. seealso::

              :ref:`with_polymorphic` - discussion of polymorphic querying techniques.

        """

        self.class_ = util.assert_arg_type(class_, type, 'class_')

        self.class_manager = None

        self._primary_key_argument = util.to_list(primary_key)
        self.non_primary = non_primary

        if order_by is not False:
            self.order_by = util.to_list(order_by)
        else:
            self.order_by = order_by

        self.always_refresh = always_refresh

        if isinstance(version_id_col, MapperProperty):
            self.version_id_prop = version_id_col
            self.version_id_col = None
        else:
            self.version_id_col = version_id_col
        if version_id_generator is False:
            self.version_id_generator = False
        elif version_id_generator is None:
            self.version_id_generator = lambda x: (x or 0) + 1
        else:
            self.version_id_generator = version_id_generator

        self.concrete = concrete
        self.single = False
        self.inherits = inherits
        self.local_table = local_table
        self.inherit_condition = inherit_condition
        self.inherit_foreign_keys = inherit_foreign_keys
        self._init_properties = properties or {}
        self._delete_orphans = []
        self.batch = batch
        self.eager_defaults = eager_defaults
        self.column_prefix = column_prefix
        self.polymorphic_on = expression._clause_element_as_expr(
                                                polymorphic_on)
        self._dependency_processors = []
        self.validators = util.immutabledict()
        self.passive_updates = passive_updates
        self.legacy_is_orphan = legacy_is_orphan
        self._clause_adapter = None
        self._requires_row_aliasing = False
        self._inherits_equated_pairs = None
        self._memoized_values = {}
        self._compiled_cache_size = _compiled_cache_size
        self._reconstructor = None
        self._deprecated_extensions = util.to_list(extension or [])

        self.allow_partial_pks = allow_partial_pks

        self._set_with_polymorphic(with_polymorphic)

        if isinstance(self.local_table, expression.SelectBase):
            raise sa_exc.InvalidRequestError(
                "When mapping against a select() construct, map against "
                "an alias() of the construct instead."
                "This because several databases don't allow a "
                "SELECT from a subquery that does not have an alias."
                )

        if self.with_polymorphic and \
                    isinstance(self.with_polymorphic[1],
                                expression.SelectBase):
            self.with_polymorphic = (self.with_polymorphic[0],
                                self.with_polymorphic[1].alias())

        # our 'polymorphic identity', a string name that when located in a
        #  result set row indicates this Mapper should be used to construct
        # the object instance for that row.
        self.polymorphic_identity = polymorphic_identity

        # a dictionary of 'polymorphic identity' names, associating those
        # names with Mappers that will be used to construct object instances
        # upon a select operation.
        if _polymorphic_map is None:
            self.polymorphic_map = {}
        else:
            self.polymorphic_map = _polymorphic_map

        if include_properties is not None:
            self.include_properties = util.to_set(include_properties)
        else:
            self.include_properties = None
        if exclude_properties:
            self.exclude_properties = util.to_set(exclude_properties)
        else:
            self.exclude_properties = None

        self.configured = False

        # prevent this mapper from being constructed
        # while a configure_mappers() is occurring (and defer a
        # configure_mappers() until construction succeeds)
        _CONFIGURE_MUTEX.acquire()
        try:
            self.dispatch._events._new_mapper_instance(class_, self)
            self._configure_inheritance()
            self._configure_legacy_instrument_class()
            self._configure_class_instrumentation()
            self._configure_listeners()
            self._configure_properties()
            self._configure_polymorphic_setter()
            self._configure_pks()
            Mapper._new_mappers = True
            self._log("constructed")
            self._expire_memoizations()
        finally:
            _CONFIGURE_MUTEX.release()

    # major attributes initialized at the classlevel so that
    # they can be Sphinx-documented.

    is_mapper = True
    """Part of the inspection API."""

    @property
    def mapper(self):
        """Part of the inspection API.

        Returns self.

        """
        return self

    @property
    def entity(self):
        """Part of the inspection API.

        Returns self.class_.

        """
        return self.class_

    local_table = None
    """The :class:`.Selectable` which this :class:`.Mapper` manages.

    Typically is an instance of :class:`.Table` or :class:`.Alias`.
    May also be ``None``.

    The "local" table is the
    selectable that the :class:`.Mapper` is directly responsible for
    managing from an attribute access and flush perspective.   For
    non-inheriting mappers, the local table is the same as the
    "mapped" table.   For joined-table inheritance mappers, local_table
    will be the particular sub-table of the overall "join" which
    this :class:`.Mapper` represents.  If this mapper is a
    single-table inheriting mapper, local_table will be ``None``.

    See also :attr:`~.Mapper.mapped_table`.

    """

    mapped_table = None
    """The :class:`.Selectable` to which this :class:`.Mapper` is mapped.

    Typically an instance of :class:`.Table`, :class:`.Join`, or
    :class:`.Alias`.

    The "mapped" table is the selectable that
    the mapper selects from during queries.   For non-inheriting
    mappers, the mapped table is the same as the "local" table.
    For joined-table inheritance mappers, mapped_table references the
    full :class:`.Join` representing full rows for this particular
    subclass.  For single-table inheritance mappers, mapped_table
    references the base table.

    See also :attr:`~.Mapper.local_table`.

    """

    inherits = None
    """References the :class:`.Mapper` which this :class:`.Mapper`
    inherits from, if any.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    configured = None
    """Represent ``True`` if this :class:`.Mapper` has been configured.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    See also :func:`.configure_mappers`.

    """

    concrete = None
    """Represent ``True`` if this :class:`.Mapper` is a concrete
    inheritance mapper.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    tables = None
    """An iterable containing the collection of :class:`.Table` objects
    which this :class:`.Mapper` is aware of.

    If the mapper is mapped to a :class:`.Join`, or an :class:`.Alias`
    representing a :class:`.Select`, the individual :class:`.Table`
    objects that comprise the full construct will be represented here.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    primary_key = None
    """An iterable containing the collection of :class:`.Column` objects
    which comprise the 'primary key' of the mapped table, from the
    perspective of this :class:`.Mapper`.

    This list is against the selectable in :attr:`~.Mapper.mapped_table`. In
    the case of inheriting mappers, some columns may be managed by a
    superclass mapper.  For example, in the case of a :class:`.Join`, the
    primary key is determined by all of the primary key columns across all
    tables referenced by the :class:`.Join`.

    The list is also not necessarily the same as the primary key column
    collection associated with the underlying tables; the :class:`.Mapper`
    features a ``primary_key`` argument that can override what the
    :class:`.Mapper` considers as primary key columns.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    class_ = None
    """The Python class which this :class:`.Mapper` maps.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    class_manager = None
    """The :class:`.ClassManager` which maintains event listeners
    and class-bound descriptors for this :class:`.Mapper`.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    single = None
    """Represent ``True`` if this :class:`.Mapper` is a single table
    inheritance mapper.

    :attr:`~.Mapper.local_table` will be ``None`` if this flag is set.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    non_primary = None
    """Represent ``True`` if this :class:`.Mapper` is a "non-primary"
    mapper, e.g. a mapper that is used only to selet rows but not for
    persistence management.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    polymorphic_on = None
    """The :class:`.Column` or SQL expression specified as the
    ``polymorphic_on`` argument
    for this :class:`.Mapper`, within an inheritance scenario.

    This attribute is normally a :class:`.Column` instance but
    may also be an expression, such as one derived from
    :func:`.cast`.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    polymorphic_map = None
    """A mapping of "polymorphic identity" identifiers mapped to
    :class:`.Mapper` instances, within an inheritance scenario.

    The identifiers can be of any type which is comparable to the
    type of column represented by :attr:`~.Mapper.polymorphic_on`.

    An inheritance chain of mappers will all reference the same
    polymorphic map object.  The object is used to correlate incoming
    result rows to target mappers.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    polymorphic_identity = None
    """Represent an identifier which is matched against the
    :attr:`~.Mapper.polymorphic_on` column during result row loading.

    Used only with inheritance, this object can be of any type which is
    comparable to the type of column represented by
    :attr:`~.Mapper.polymorphic_on`.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    base_mapper = None
    """The base-most :class:`.Mapper` in an inheritance chain.

    In a non-inheriting scenario, this attribute will always be this
    :class:`.Mapper`.   In an inheritance scenario, it references
    the :class:`.Mapper` which is parent to all other :class:`.Mapper`
    objects in the inheritance chain.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    columns = None
    """A collection of :class:`.Column` or other scalar expression
    objects maintained by this :class:`.Mapper`.

    The collection behaves the same as that of the ``c`` attribute on
    any :class:`.Table` object, except that only those columns included in
    this mapping are present, and are keyed based on the attribute name
    defined in the mapping, not necessarily the ``key`` attribute of the
    :class:`.Column` itself.   Additionally, scalar expressions mapped
    by :func:`.column_property` are also present here.

    This is a *read only* attribute determined during mapper construction.
    Behavior is undefined if directly modified.

    """

    validators = None
    """An immutable dictionary of attributes which have been decorated
    using the :func:`~.orm.validates` decorator.

    The dictionary contains string attribute names as keys
    mapped to the actual validation method.

    """

    c = None
    """A synonym for :attr:`~.Mapper.columns`."""

    @util.memoized_property
    def _path_registry(self):
        return PathRegistry.per_mapper(self)

    def _configure_inheritance(self):
        """Configure settings related to inherting and/or inherited mappers
        being present."""

        # a set of all mappers which inherit from this one.
        self._inheriting_mappers = util.WeakSequence()

        if self.inherits:
            if isinstance(self.inherits, type):
                self.inherits = class_mapper(self.inherits, configure=False)
            if not issubclass(self.class_, self.inherits.class_):
                raise sa_exc.ArgumentError(
                        "Class '%s' does not inherit from '%s'" %
                        (self.class_.__name__, self.inherits.class_.__name__))
            if self.non_primary != self.inherits.non_primary:
                np = not self.non_primary and "primary" or "non-primary"
                raise sa_exc.ArgumentError(
                        "Inheritance of %s mapper for class '%s' is "
                        "only allowed from a %s mapper" %
                        (np, self.class_.__name__, np))
            # inherit_condition is optional.
            if self.local_table is None:
                self.local_table = self.inherits.local_table
                self.mapped_table = self.inherits.mapped_table
                self.single = True
            elif not self.local_table is self.inherits.local_table:
                if self.concrete:
                    self.mapped_table = self.local_table
                    for mapper in self.iterate_to_root():
                        if mapper.polymorphic_on is not None:
                            mapper._requires_row_aliasing = True
                else:
                    if self.inherit_condition is None:
                        # figure out inherit condition from our table to the
                        # immediate table of the inherited mapper, not its
                        # full table which could pull in other stuff we dont
                        # want (allows test/inheritance.InheritTest4 to pass)
                        self.inherit_condition = sql_util.join_condition(
                                                    self.inherits.local_table,
                                                    self.local_table)
                    self.mapped_table = sql.join(
                                                self.inherits.mapped_table,
                                                self.local_table,
                                                self.inherit_condition)

                    fks = util.to_set(self.inherit_foreign_keys)
                    self._inherits_equated_pairs = sql_util.criterion_as_pairs(
                                                self.mapped_table.onclause,
                                                consider_as_foreign_keys=fks)
            else:
                self.mapped_table = self.local_table

            if self.polymorphic_identity is not None and not self.concrete:
                self._identity_class = self.inherits._identity_class
            else:
                self._identity_class = self.class_

            if self.version_id_col is None:
                self.version_id_col = self.inherits.version_id_col
                self.version_id_generator = self.inherits.version_id_generator
            elif self.inherits.version_id_col is not None and \
                self.version_id_col is not self.inherits.version_id_col:
                util.warn(
                    "Inheriting version_id_col '%s' does not match inherited "
                    "version_id_col '%s' and will not automatically populate "
                    "the inherited versioning column. "
                    "version_id_col should only be specified on "
                    "the base-most mapper that includes versioning." %
                    (self.version_id_col.description,
                    self.inherits.version_id_col.description)
                )

            if self.order_by is False and \
                        not self.concrete and \
                        self.inherits.order_by is not False:
                self.order_by = self.inherits.order_by

            self.polymorphic_map = self.inherits.polymorphic_map
            self.batch = self.inherits.batch
            self.inherits._inheriting_mappers.append(self)
            self.base_mapper = self.inherits.base_mapper
            self.passive_updates = self.inherits.passive_updates
            self._all_tables = self.inherits._all_tables

            if self.polymorphic_identity is not None:
                self.polymorphic_map[self.polymorphic_identity] = self

        else:
            self._all_tables = set()
            self.base_mapper = self
            self.mapped_table = self.local_table
            if self.polymorphic_identity is not None:
                self.polymorphic_map[self.polymorphic_identity] = self
            self._identity_class = self.class_

        if self.mapped_table is None:
            raise sa_exc.ArgumentError(
                    "Mapper '%s' does not have a mapped_table specified."
                    % self)

    def _set_with_polymorphic(self, with_polymorphic):
        if with_polymorphic == '*':
            self.with_polymorphic = ('*', None)
        elif isinstance(with_polymorphic, (tuple, list)):
            if isinstance(with_polymorphic[0], util.string_types + (tuple, list)):
                self.with_polymorphic = with_polymorphic
            else:
                self.with_polymorphic = (with_polymorphic, None)
        elif with_polymorphic is not None:
            raise sa_exc.ArgumentError("Invalid setting for with_polymorphic")
        else:
            self.with_polymorphic = None

        if isinstance(self.local_table, expression.SelectBase):
            raise sa_exc.InvalidRequestError(
                "When mapping against a select() construct, map against "
                "an alias() of the construct instead."
                "This because several databases don't allow a "
                "SELECT from a subquery that does not have an alias."
                )

        if self.with_polymorphic and \
                    isinstance(self.with_polymorphic[1],
                                expression.SelectBase):
            self.with_polymorphic = (self.with_polymorphic[0],
                                self.with_polymorphic[1].alias())
        if self.configured:
            self._expire_memoizations()

    def _set_concrete_base(self, mapper):
        """Set the given :class:`.Mapper` as the 'inherits' for this
        :class:`.Mapper`, assuming this :class:`.Mapper` is concrete
        and does not already have an inherits."""

        assert self.concrete
        assert not self.inherits
        assert isinstance(mapper, Mapper)
        self.inherits = mapper
        self.inherits.polymorphic_map.update(self.polymorphic_map)
        self.polymorphic_map = self.inherits.polymorphic_map
        for mapper in self.iterate_to_root():
            if mapper.polymorphic_on is not None:
                mapper._requires_row_aliasing = True
        self.batch = self.inherits.batch
        for mp in self.self_and_descendants:
            mp.base_mapper = self.inherits.base_mapper
        self.inherits._inheriting_mappers.append(self)
        self.passive_updates = self.inherits.passive_updates
        self._all_tables = self.inherits._all_tables
        for key, prop in mapper._props.items():
            if key not in self._props and \
                not self._should_exclude(key, key, local=False,
                                        column=None):
                self._adapt_inherited_property(key, prop, False)

    def _set_polymorphic_on(self, polymorphic_on):
        self.polymorphic_on = polymorphic_on
        self._configure_polymorphic_setter(True)

    def _configure_legacy_instrument_class(self):

        if self.inherits:
            self.dispatch._update(self.inherits.dispatch)
            super_extensions = set(
                        chain(*[m._deprecated_extensions
                        for m in self.inherits.iterate_to_root()]))
        else:
            super_extensions = set()

        for ext in self._deprecated_extensions:
            if ext not in super_extensions:
                ext._adapt_instrument_class(self, ext)

    def _configure_listeners(self):
        if self.inherits:
            super_extensions = set(
                        chain(*[m._deprecated_extensions
                        for m in self.inherits.iterate_to_root()]))
        else:
            super_extensions = set()

        for ext in self._deprecated_extensions:
            if ext not in super_extensions:
                ext._adapt_listener(self, ext)

    def _configure_class_instrumentation(self):
        """If this mapper is to be a primary mapper (i.e. the
        non_primary flag is not set), associate this Mapper with the
        given class_ and entity name.

        Subsequent calls to ``class_mapper()`` for the class_/entity
        name combination will return this mapper.  Also decorate the
        `__init__` method on the mapped class to include optional
        auto-session attachment logic.

        """
        manager = attributes.manager_of_class(self.class_)

        if self.non_primary:
            if not manager or not manager.is_mapped:
                raise sa_exc.InvalidRequestError(
                    "Class %s has no primary mapper configured.  Configure "
                    "a primary mapper first before setting up a non primary "
                    "Mapper." % self.class_)
            self.class_manager = manager
            self._identity_class = manager.mapper._identity_class
            _mapper_registry[self] = True
            return

        if manager is not None:
            assert manager.class_ is self.class_
            if manager.is_mapped:
                raise sa_exc.ArgumentError(
                    "Class '%s' already has a primary mapper defined. "
                    "Use non_primary=True to "
                    "create a non primary Mapper.  clear_mappers() will "
                    "remove *all* current mappers from all classes." %
                    self.class_)
            #else:
                # a ClassManager may already exist as
                # ClassManager.instrument_attribute() creates
                # new managers for each subclass if they don't yet exist.

        _mapper_registry[self] = True

        self.dispatch.instrument_class(self, self.class_)

        if manager is None:
            manager = instrumentation.register_class(self.class_)

        self.class_manager = manager

        manager.mapper = self
        manager.deferred_scalar_loader = util.partial(
                            loading.load_scalar_attributes, self)

        # The remaining members can be added by any mapper,
        # e_name None or not.
        if manager.info.get(_INSTRUMENTOR, False):
            return

        event.listen(manager, 'first_init', _event_on_first_init, raw=True)
        event.listen(manager, 'init', _event_on_init, raw=True)
        event.listen(manager, 'resurrect', _event_on_resurrect, raw=True)

        for key, method in util.iterate_attributes(self.class_):
            if isinstance(method, types.FunctionType):
                if hasattr(method, '__sa_reconstructor__'):
                    self._reconstructor = method
                    event.listen(manager, 'load', _event_on_load, raw=True)
                elif hasattr(method, '__sa_validators__'):
                    include_removes = getattr(method,
                                            "__sa_include_removes__", False)
                    for name in method.__sa_validators__:
                        self.validators = self.validators.union(
                            {name: (method, include_removes)}
                        )

        manager.info[_INSTRUMENTOR] = self


    @classmethod
    def _configure_all(cls):
        """Class-level path to the :func:`.configure_mappers` call.
        """
        configure_mappers()

    def dispose(self):
        # Disable any attribute-based compilation.
        self.configured = True

        if hasattr(self, '_configure_failed'):
            del self._configure_failed

        if not self.non_primary and \
            self.class_manager is not None and \
            self.class_manager.is_mapped and \
                self.class_manager.mapper is self:
            instrumentation.unregister_class(self.class_)

    def _configure_pks(self):

        self.tables = sql_util.find_tables(self.mapped_table)

        self._pks_by_table = {}
        self._cols_by_table = {}

        all_cols = util.column_set(chain(*[
                                col.proxy_set for col in
                                self._columntoproperty]))

        pk_cols = util.column_set(c for c in all_cols if c.primary_key)

        # identify primary key columns which are also mapped by this mapper.
        tables = set(self.tables + [self.mapped_table])
        self._all_tables.update(tables)
        for t in tables:
            if t.primary_key and pk_cols.issuperset(t.primary_key):
                # ordering is important since it determines the ordering of
                # mapper.primary_key (and therefore query.get())
                self._pks_by_table[t] = \
                                    util.ordered_column_set(t.primary_key).\
                                    intersection(pk_cols)
            self._cols_by_table[t] = \
                                    util.ordered_column_set(t.c).\
                                    intersection(all_cols)

        # determine cols that aren't expressed within our tables; mark these
        # as "read only" properties which are refreshed upon INSERT/UPDATE
        self._readonly_props = set(
            self._columntoproperty[col]
            for col in self._columntoproperty
            if not hasattr(col, 'table') or
            col.table not in self._cols_by_table)

        # if explicit PK argument sent, add those columns to the
        # primary key mappings
        if self._primary_key_argument:
            for k in self._primary_key_argument:
                if k.table not in self._pks_by_table:
                    self._pks_by_table[k.table] = util.OrderedSet()
                self._pks_by_table[k.table].add(k)

        # otherwise, see that we got a full PK for the mapped table
        elif self.mapped_table not in self._pks_by_table or \
                    len(self._pks_by_table[self.mapped_table]) == 0:
                raise sa_exc.ArgumentError(
                        "Mapper %s could not assemble any primary "
                        "key columns for mapped table '%s'" %
                        (self, self.mapped_table.description))
        elif self.local_table not in self._pks_by_table and \
            isinstance(self.local_table, schema.Table):
            util.warn("Could not assemble any primary "
                        "keys for locally mapped table '%s' - "
                        "no rows will be persisted in this Table."
                        % self.local_table.description)

        if self.inherits and \
                not self.concrete and \
                not self._primary_key_argument:
            # if inheriting, the "primary key" for this mapper is
            # that of the inheriting (unless concrete or explicit)
            self.primary_key = self.inherits.primary_key
        else:
            # determine primary key from argument or mapped_table pks -
            # reduce to the minimal set of columns
            if self._primary_key_argument:
                primary_key = sql_util.reduce_columns(
                    [self.mapped_table.corresponding_column(c) for c in
                    self._primary_key_argument],
                    ignore_nonexistent_tables=True)
            else:
                primary_key = sql_util.reduce_columns(
                                self._pks_by_table[self.mapped_table],
                                ignore_nonexistent_tables=True)

            if len(primary_key) == 0:
                raise sa_exc.ArgumentError(
                    "Mapper %s could not assemble any primary "
                    "key columns for mapped table '%s'" %
                    (self, self.mapped_table.description))

            self.primary_key = tuple(primary_key)
            self._log("Identified primary key columns: %s", primary_key)

    def _configure_properties(self):

        # Column and other ClauseElement objects which are mapped
        self.columns = self.c = util.OrderedProperties()

        # object attribute names mapped to MapperProperty objects
        self._props = util.OrderedDict()

        # table columns mapped to lists of MapperProperty objects
        # using a list allows a single column to be defined as
        # populating multiple object attributes
        self._columntoproperty = _ColumnMapping(self)

        # load custom properties
        if self._init_properties:
            for key, prop in self._init_properties.items():
                self._configure_property(key, prop, False)

        # pull properties from the inherited mapper if any.
        if self.inherits:
            for key, prop in self.inherits._props.items():
                if key not in self._props and \
                    not self._should_exclude(key, key, local=False,
                                            column=None):
                    self._adapt_inherited_property(key, prop, False)

        # create properties for each column in the mapped table,
        # for those columns which don't already map to a property
        for column in self.mapped_table.columns:
            if column in self._columntoproperty:
                continue

            column_key = (self.column_prefix or '') + column.key

            if self._should_exclude(
                            column.key, column_key,
                             local=self.local_table.c.contains_column(column),
                             column=column
                            ):
                continue

            # adjust the "key" used for this column to that
            # of the inheriting mapper
            for mapper in self.iterate_to_root():
                if column in mapper._columntoproperty:
                    column_key = mapper._columntoproperty[column].key

            self._configure_property(column_key,
                                    column,
                                    init=False,
                                    setparent=True)

    def _configure_polymorphic_setter(self, init=False):
        """Configure an attribute on the mapper representing the
        'polymorphic_on' column, if applicable, and not
        already generated by _configure_properties (which is typical).

        Also create a setter function which will assign this
        attribute to the value of the 'polymorphic_identity'
        upon instance construction, also if applicable.  This
        routine will run when an instance is created.

        """
        setter = False

        if self.polymorphic_on is not None:
            setter = True

            if isinstance(self.polymorphic_on, util.string_types):
                # polymorphic_on specified as as string - link
                # it to mapped ColumnProperty
                try:
                    self.polymorphic_on = self._props[self.polymorphic_on]
                except KeyError:
                    raise sa_exc.ArgumentError(
                                "Can't determine polymorphic_on "
                                "value '%s' - no attribute is "
                                "mapped to this name." % self.polymorphic_on)

            if self.polymorphic_on in self._columntoproperty:
                # polymorphic_on is a column that is already mapped
                # to a ColumnProperty
                prop = self._columntoproperty[self.polymorphic_on]
                polymorphic_key = prop.key
                self.polymorphic_on = prop.columns[0]
                polymorphic_key = prop.key
            elif isinstance(self.polymorphic_on, MapperProperty):
                # polymorphic_on is directly a MapperProperty,
                # ensure it's a ColumnProperty
                if not isinstance(self.polymorphic_on,
                                    properties.ColumnProperty):
                    raise sa_exc.ArgumentError(
                            "Only direct column-mapped "
                            "property or SQL expression "
                            "can be passed for polymorphic_on")
                prop = self.polymorphic_on
                self.polymorphic_on = prop.columns[0]
                polymorphic_key = prop.key
            elif not expression._is_column(self.polymorphic_on):
                # polymorphic_on is not a Column and not a ColumnProperty;
                # not supported right now.
                raise sa_exc.ArgumentError(
                    "Only direct column-mapped "
                    "property or SQL expression "
                    "can be passed for polymorphic_on"
                )
            else:
                # polymorphic_on is a Column or SQL expression and
                # doesn't appear to be mapped. this means it can be 1.
                # only present in the with_polymorphic selectable or
                # 2. a totally standalone SQL expression which we'd
                # hope is compatible with this mapper's mapped_table
                col = self.mapped_table.corresponding_column(
                            self.polymorphic_on)
                if col is None:
                    # polymorphic_on doesn't derive from any
                    # column/expression isn't present in the mapped
                    # table. we will make a "hidden" ColumnProperty
                    # for it. Just check that if it's directly a
                    # schema.Column and we have with_polymorphic, it's
                    # likely a user error if the schema.Column isn't
                    # represented somehow in either mapped_table or
                    # with_polymorphic.   Otherwise as of 0.7.4 we
                    # just go with it and assume the user wants it
                    # that way (i.e. a CASE statement)
                    setter = False
                    instrument = False
                    col = self.polymorphic_on
                    if isinstance(col, schema.Column) and (
                        self.with_polymorphic is  None or \
                        self.with_polymorphic[1].\
                            corresponding_column(col) is None
                        ):
                        raise sa_exc.InvalidRequestError(
                            "Could not map polymorphic_on column "
                            "'%s' to the mapped table - polymorphic "
                            "loads will not function properly"
                                 % col.description)
                else:
                    # column/expression that polymorphic_on derives from
                    # is present in our mapped table
                    # and is probably mapped, but polymorphic_on itself
                    # is not.  This happens when
                    # the polymorphic_on is only directly present in the
                    # with_polymorphic selectable, as when use
                    # polymorphic_union.
                    # we'll make a separate ColumnProperty for it.
                    instrument = True
                key = getattr(col, 'key', None)
                if key:
                    if self._should_exclude(col.key, col.key, False, col):
                        raise sa_exc.InvalidRequestError(
                        "Cannot exclude or override the "
                        "discriminator column %r" %
                        col.key)
                else:
                    self.polymorphic_on = col = \
                                col.label("_sa_polymorphic_on")
                    key = col.key

                self._configure_property(
                                key,
                                properties.ColumnProperty(col,
                                                    _instrument=instrument),
                                init=init, setparent=True)
                polymorphic_key = key
        else:
            # no polymorphic_on was set.
            # check inheriting mappers for one.
            for mapper in self.iterate_to_root():
                # determine if polymorphic_on of the parent
                # should be propagated here.   If the col
                # is present in our mapped table, or if our mapped
                # table is the same as the parent (i.e. single table
                # inheritance), we can use it
                if mapper.polymorphic_on is not None:
                    if self.mapped_table is mapper.mapped_table:
                        self.polymorphic_on = mapper.polymorphic_on
                    else:
                        self.polymorphic_on = \
                                self.mapped_table.corresponding_column(
                                                    mapper.polymorphic_on)
                    # we can use the parent mapper's _set_polymorphic_identity
                    # directly; it ensures the polymorphic_identity of the
                    # instance's mapper is used so is portable to subclasses.
                    if self.polymorphic_on is not None:
                        self._set_polymorphic_identity = \
                            mapper._set_polymorphic_identity
                        self._validate_polymorphic_identity = \
                            mapper._validate_polymorphic_identity
                    else:
                        self._set_polymorphic_identity = None
                    return

        if setter:
            def _set_polymorphic_identity(state):
                dict_ = state.dict
                state.get_impl(polymorphic_key).set(state, dict_,
                        state.manager.mapper.polymorphic_identity, None)

            def _validate_polymorphic_identity(mapper, state, dict_):
                if polymorphic_key in dict_ and \
                    dict_[polymorphic_key] not in \
                    mapper._acceptable_polymorphic_identities:
                    util.warn(
                                "Flushing object %s with "
                                "incompatible polymorphic identity %r; the "
                                "object may not refresh and/or load correctly" % (
                                        state_str(state),
                                        dict_[polymorphic_key]
                                    )
                            )

            self._set_polymorphic_identity = _set_polymorphic_identity
            self._validate_polymorphic_identity = _validate_polymorphic_identity
        else:
            self._set_polymorphic_identity = None


    _validate_polymorphic_identity = None

    @_memoized_configured_property
    def _version_id_prop(self):
        if self.version_id_col is not None:
            return self._columntoproperty[self.version_id_col]
        else:
            return None

    @_memoized_configured_property
    def _acceptable_polymorphic_identities(self):
        identities = set()

        stack = deque([self])
        while stack:
            item = stack.popleft()
            if item.mapped_table is self.mapped_table:
                identities.add(item.polymorphic_identity)
                stack.extend(item._inheriting_mappers)

        return identities

    def _adapt_inherited_property(self, key, prop, init):
        if not self.concrete:
            self._configure_property(key, prop, init=False, setparent=False)
        elif key not in self._props:
            self._configure_property(
                            key,
                            properties.ConcreteInheritedProperty(),
                            init=init, setparent=True)

    def _configure_property(self, key, prop, init=True, setparent=True):
        self._log("_configure_property(%s, %s)", key, prop.__class__.__name__)

        if not isinstance(prop, MapperProperty):
            prop = self._property_from_column(key, prop)

        if isinstance(prop, properties.ColumnProperty):
            col = self.mapped_table.corresponding_column(prop.columns[0])

            # if the column is not present in the mapped table,
            # test if a column has been added after the fact to the
            # parent table (or their parent, etc.) [ticket:1570]
            if col is None and self.inherits:
                path = [self]
                for m in self.inherits.iterate_to_root():
                    col = m.local_table.corresponding_column(prop.columns[0])
                    if col is not None:
                        for m2 in path:
                            m2.mapped_table._reset_exported()
                        col = self.mapped_table.corresponding_column(
                                                prop.columns[0])
                        break
                    path.append(m)

            # subquery expression, column not present in the mapped
            # selectable.
            if col is None:
                col = prop.columns[0]

                # column is coming in after _readonly_props was
                # initialized; check for 'readonly'
                if hasattr(self, '_readonly_props') and \
                    (not hasattr(col, 'table') or
                        col.table not in self._cols_by_table):
                        self._readonly_props.add(prop)

            else:
                # if column is coming in after _cols_by_table was
                # initialized, ensure the col is in the right set
                if hasattr(self, '_cols_by_table') and \
                                    col.table in self._cols_by_table and \
                                    col not in self._cols_by_table[col.table]:
                    self._cols_by_table[col.table].add(col)

            # if this properties.ColumnProperty represents the "polymorphic
            # discriminator" column, mark it.  We'll need this when rendering
            # columns in SELECT statements.
            if not hasattr(prop, '_is_polymorphic_discriminator'):
                prop._is_polymorphic_discriminator = \
                                    (col is self.polymorphic_on or
                                    prop.columns[0] is self.polymorphic_on)

            self.columns[key] = col
            for col in prop.columns + prop._orig_columns:
                for col in col.proxy_set:
                    self._columntoproperty[col] = prop

        prop.key = key

        if setparent:
            prop.set_parent(self, init)

        if key in self._props and \
                getattr(self._props[key], '_mapped_by_synonym', False):
            syn = self._props[key]._mapped_by_synonym
            raise sa_exc.ArgumentError(
                        "Can't call map_column=True for synonym %r=%r, "
                        "a ColumnProperty already exists keyed to the name "
                        "%r for column %r" % (syn, key, key, syn)
                    )

        if key in self._props and \
                not isinstance(prop, properties.ColumnProperty) and \
                not isinstance(self._props[key], properties.ColumnProperty):
            util.warn("Property %s on %s being replaced with new "
                            "property %s; the old property will be discarded" % (
                            self._props[key],
                            self,
                            prop,
                        ))

        self._props[key] = prop

        if not self.non_primary:
            prop.instrument_class(self)

        for mapper in self._inheriting_mappers:
            mapper._adapt_inherited_property(key, prop, init)

        if init:
            prop.init()
            prop.post_instrument_class(self)

        if self.configured:
            self._expire_memoizations()

    def _property_from_column(self, key, prop):
        """generate/update a :class:`.ColumnProprerty` given a
        :class:`.Column` object. """

        # we were passed a Column or a list of Columns;
        # generate a properties.ColumnProperty
        columns = util.to_list(prop)
        column = columns[0]
        if not expression._is_column(column):
            raise sa_exc.ArgumentError(
                    "%s=%r is not an instance of MapperProperty or Column"
                    % (key, prop))

        prop = self._props.get(key, None)

        if isinstance(prop, properties.ColumnProperty):
            if prop.parent is self:
                raise sa_exc.InvalidRequestError(
                        "Implicitly combining column %s with column "
                        "%s under attribute '%s'.  Please configure one "
                        "or more attributes for these same-named columns "
                        "explicitly."
                         % (prop.columns[-1], column, key))

            # existing properties.ColumnProperty from an inheriting
            # mapper. make a copy and append our column to it
            prop = prop.copy()
            prop.columns.insert(0, column)
            self._log("inserting column to existing list "
                        "in properties.ColumnProperty %s" % (key))
            return prop
        elif prop is None or isinstance(prop,
                                properties.ConcreteInheritedProperty):
            mapped_column = []
            for c in columns:
                mc = self.mapped_table.corresponding_column(c)
                if mc is None:
                    mc = self.local_table.corresponding_column(c)
                    if mc is not None:
                        # if the column is in the local table but not the
                        # mapped table, this corresponds to adding a
                        # column after the fact to the local table.
                        # [ticket:1523]
                        self.mapped_table._reset_exported()
                    mc = self.mapped_table.corresponding_column(c)
                    if mc is None:
                        raise sa_exc.ArgumentError(
                        "When configuring property '%s' on %s, "
                        "column '%s' is not represented in the mapper's "
                        "table. Use the `column_property()` function to "
                        "force this column to be mapped as a read-only "
                        "attribute." % (key, self, c))
                mapped_column.append(mc)
            return properties.ColumnProperty(*mapped_column)
        else:
            raise sa_exc.ArgumentError(
                "WARNING: when configuring property '%s' on %s, "
                "column '%s' conflicts with property '%r'. "
                "To resolve this, map the column to the class under a "
                "different name in the 'properties' dictionary.  Or, "
                "to remove all awareness of the column entirely "
                "(including its availability as a foreign key), "
                "use the 'include_properties' or 'exclude_properties' "
                "mapper arguments to control specifically which table "
                "columns get mapped." %
                (key, self, column.key, prop))

    def _post_configure_properties(self):
        """Call the ``init()`` method on all ``MapperProperties``
        attached to this mapper.

        This is a deferred configuration step which is intended
        to execute once all mappers have been constructed.

        """

        self._log("_post_configure_properties() started")
        l = [(key, prop) for key, prop in self._props.items()]
        for key, prop in l:
            self._log("initialize prop %s", key)

            if prop.parent is self and not prop._configure_started:
                prop.init()

            if prop._configure_finished:
                prop.post_instrument_class(self)

        self._log("_post_configure_properties() complete")
        self.configured = True

    def add_properties(self, dict_of_properties):
        """Add the given dictionary of properties to this mapper,
        using `add_property`.

        """
        for key, value in dict_of_properties.items():
            self.add_property(key, value)

    def add_property(self, key, prop):
        """Add an individual MapperProperty to this mapper.

        If the mapper has not been configured yet, just adds the
        property to the initial properties dictionary sent to the
        constructor.  If this Mapper has already been configured, then
        the given MapperProperty is configured immediately.

        """
        self._init_properties[key] = prop
        self._configure_property(key, prop, init=self.configured)

    def _expire_memoizations(self):
        for mapper in self.iterate_to_root():
            _memoized_configured_property.expire_instance(mapper)

    @property
    def _log_desc(self):
        return "(" + self.class_.__name__ + \
            "|" + \
            (self.local_table is not None and
                self.local_table.description or
                str(self.local_table)) +\
            (self.non_primary and
            "|non-primary" or "") + ")"

    def _log(self, msg, *args):
        self.logger.info(
            "%s " + msg, *((self._log_desc,) + args)
        )

    def _log_debug(self, msg, *args):
        self.logger.debug(
            "%s " + msg, *((self._log_desc,) + args)
        )

    def __repr__(self):
        return '<Mapper at 0x%x; %s>' % (
            id(self), self.class_.__name__)

    def __str__(self):
        return "Mapper|%s|%s%s" % (
            self.class_.__name__,
            self.local_table is not None and
            self.local_table.description or None,
            self.non_primary and "|non-primary" or ""
        )

    def _is_orphan(self, state):
        orphan_possible = False
        for mapper in self.iterate_to_root():
            for (key, cls) in mapper._delete_orphans:
                orphan_possible = True

                has_parent = attributes.manager_of_class(cls).has_parent(
                        state, key, optimistic=state.has_identity)

                if self.legacy_is_orphan and has_parent:
                    return False
                elif not self.legacy_is_orphan and not has_parent:
                    return True

        if self.legacy_is_orphan:
            return orphan_possible
        else:
            return False

    def has_property(self, key):
        return key in self._props

    def get_property(self, key, _configure_mappers=True):
        """return a MapperProperty associated with the given key.
        """

        if _configure_mappers and Mapper._new_mappers:
            configure_mappers()

        try:
            return self._props[key]
        except KeyError:
            raise sa_exc.InvalidRequestError(
                    "Mapper '%s' has no property '%s'" % (self, key))

    def get_property_by_column(self, column):
        """Given a :class:`.Column` object, return the
        :class:`.MapperProperty` which maps this column."""

        return self._columntoproperty[column]

    @property
    def iterate_properties(self):
        """return an iterator of all MapperProperty objects."""
        if Mapper._new_mappers:
            configure_mappers()
        return iter(self._props.values())

    def _mappers_from_spec(self, spec, selectable):
        """given a with_polymorphic() argument, return the set of mappers it
        represents.

        Trims the list of mappers to just those represented within the given
        selectable, if present. This helps some more legacy-ish mappings.

        """
        if spec == '*':
            mappers = list(self.self_and_descendants)
        elif spec:
            mappers = set()
            for m in util.to_list(spec):
                m = _class_to_mapper(m)
                if not m.isa(self):
                    raise sa_exc.InvalidRequestError(
                                "%r does not inherit from %r" %
                                (m, self))

                if selectable is None:
                    mappers.update(m.iterate_to_root())
                else:
                    mappers.add(m)
            mappers = [m for m in self.self_and_descendants if m in mappers]
        else:
            mappers = []

        if selectable is not None:
            tables = set(sql_util.find_tables(selectable,
                            include_aliases=True))
            mappers = [m for m in mappers if m.local_table in tables]
        return mappers

    def _selectable_from_mappers(self, mappers, innerjoin):
        """given a list of mappers (assumed to be within this mapper's
        inheritance hierarchy), construct an outerjoin amongst those mapper's
        mapped tables.

        """
        from_obj = self.mapped_table
        for m in mappers:
            if m is self:
                continue
            if m.concrete:
                raise sa_exc.InvalidRequestError(
                        "'with_polymorphic()' requires 'selectable' argument "
                        "when concrete-inheriting mappers are used.")
            elif not m.single:
                if innerjoin:
                    from_obj = from_obj.join(m.local_table,
                                                m.inherit_condition)
                else:
                    from_obj = from_obj.outerjoin(m.local_table,
                                                m.inherit_condition)

        return from_obj

    @_memoized_configured_property
    def _single_table_criterion(self):
        if self.single and \
            self.inherits and \
            self.polymorphic_on is not None:
            return self.polymorphic_on.in_(
                m.polymorphic_identity
                for m in self.self_and_descendants)
        else:
            return None

    @_memoized_configured_property
    def _with_polymorphic_mappers(self):
        if Mapper._new_mappers:
            configure_mappers()
        if not self.with_polymorphic:
            return []
        return self._mappers_from_spec(*self.with_polymorphic)

    @_memoized_configured_property
    def _with_polymorphic_selectable(self):
        if not self.with_polymorphic:
            return self.mapped_table

        spec, selectable = self.with_polymorphic
        if selectable is not None:
            return selectable
        else:
            return self._selectable_from_mappers(
                            self._mappers_from_spec(spec, selectable),
                            False)

    with_polymorphic_mappers = _with_polymorphic_mappers
    """The list of :class:`.Mapper` objects included in the
    default "polymorphic" query.

    """

    @property
    def selectable(self):
        """The :func:`.select` construct this :class:`.Mapper` selects from
        by default.

        Normally, this is equivalent to :attr:`.mapped_table`, unless
        the ``with_polymorphic`` feature is in use, in which case the
        full "polymorphic" selectable is returned.

        """
        return self._with_polymorphic_selectable

    def _with_polymorphic_args(self, spec=None, selectable=False,
                                innerjoin=False):
        if self.with_polymorphic:
            if not spec:
                spec = self.with_polymorphic[0]
            if selectable is False:
                selectable = self.with_polymorphic[1]
        elif selectable is False:
            selectable = None
        mappers = self._mappers_from_spec(spec, selectable)
        if selectable is not None:
            return mappers, selectable
        else:
            return mappers, self._selectable_from_mappers(mappers,
                                innerjoin)

    @_memoized_configured_property
    def _polymorphic_properties(self):
        return list(self._iterate_polymorphic_properties(
            self._with_polymorphic_mappers))


    def _iterate_polymorphic_properties(self, mappers=None):
        """Return an iterator of MapperProperty objects which will render into
        a SELECT."""
        if mappers is None:
            mappers = self._with_polymorphic_mappers

        if not mappers:
            for c in self.iterate_properties:
                yield c
        else:
            # in the polymorphic case, filter out discriminator columns
            # from other mappers, as these are sometimes dependent on that
            # mapper's polymorphic selectable (which we don't want rendered)
            for c in util.unique_list(
                chain(*[
                        list(mapper.iterate_properties) for mapper in
                        [self] + mappers
                    ])
            ):
                if getattr(c, '_is_polymorphic_discriminator', False) and \
                        (self.polymorphic_on is None or
                        c.columns[0] is not self.polymorphic_on):
                        continue
                yield c

    @util.memoized_property
    def attrs(self):
        """A namespace of all :class:`.MapperProperty` objects
        associated this mapper.

        This is an object that provides each property based on
        its key name.  For instance, the mapper for a
        ``User`` class which has ``User.name`` attribute would
        provide ``mapper.attrs.name``, which would be the
        :class:`.ColumnProperty` representing the ``name``
        column.   The namespace object can also be iterated,
        which would yield each :class:`.MapperProperty`.

        :class:`.Mapper` has several pre-filtered views
        of this attribute which limit the types of properties
        returned, inclding :attr:`.synonyms`, :attr:`.column_attrs`,
        :attr:`.relationships`, and :attr:`.composites`.

        .. seealso::

            :attr:`.Mapper.all_orm_descriptors`

        """
        if Mapper._new_mappers:
            configure_mappers()
        return util.ImmutableProperties(self._props)

    @util.memoized_property
    def all_orm_descriptors(self):
        """A namespace of all :class:`._InspectionAttr` attributes associated
        with the mapped class.

        These attributes are in all cases Python :term:`descriptors` associated
        with the mapped class or its superclasses.

        This namespace includes attributes that are mapped to the class
        as well as attributes declared by extension modules.
        It includes any Python descriptor type that inherits from
        :class:`._InspectionAttr`.  This includes :class:`.QueryableAttribute`,
        as well as extension types such as :class:`.hybrid_property`,
        :class:`.hybrid_method` and :class:`.AssociationProxy`.

        To distinguish between mapped attributes and extension attributes,
        the attribute :attr:`._InspectionAttr.extension_type` will refer
        to a constant that distinguishes between different extension types.

        When dealing with a :class:`.QueryableAttribute`, the
        :attr:`.QueryableAttribute.property` attribute refers to the
        :class:`.MapperProperty` property, which is what you get when referring
        to the collection of mapped properties via :attr:`.Mapper.attrs`.

        .. versionadded:: 0.8.0

        .. seealso::

            :attr:`.Mapper.attrs`

        """
        return util.ImmutableProperties(
                            dict(self.class_manager._all_sqla_attributes()))

    @_memoized_configured_property
    def synonyms(self):
        """Return a namespace of all :class:`.SynonymProperty`
        properties maintained by this :class:`.Mapper`.

        See also:

        :attr:`.Mapper.attrs` - namespace of all :class:`.MapperProperty`
        objects.

        """
        return self._filter_properties(properties.SynonymProperty)

    @_memoized_configured_property
    def column_attrs(self):
        """Return a namespace of all :class:`.ColumnProperty`
        properties maintained by this :class:`.Mapper`.

        See also:

        :attr:`.Mapper.attrs` - namespace of all :class:`.MapperProperty`
        objects.

        """
        return self._filter_properties(properties.ColumnProperty)

    @_memoized_configured_property
    def relationships(self):
        """Return a namespace of all :class:`.RelationshipProperty`
        properties maintained by this :class:`.Mapper`.

        See also:

        :attr:`.Mapper.attrs` - namespace of all :class:`.MapperProperty`
        objects.

        """
        return self._filter_properties(properties.RelationshipProperty)

    @_memoized_configured_property
    def composites(self):
        """Return a namespace of all :class:`.CompositeProperty`
        properties maintained by this :class:`.Mapper`.

        See also:

        :attr:`.Mapper.attrs` - namespace of all :class:`.MapperProperty`
        objects.

        """
        return self._filter_properties(properties.CompositeProperty)

    def _filter_properties(self, type_):
        if Mapper._new_mappers:
            configure_mappers()
        return util.ImmutableProperties(util.OrderedDict(
            (k, v) for k, v in self._props.items()
            if isinstance(v, type_)
        ))

    @_memoized_configured_property
    def _get_clause(self):
        """create a "get clause" based on the primary key.  this is used
        by query.get() and many-to-one lazyloads to load this item
        by primary key.

        """
        params = [(primary_key, sql.bindparam(None, type_=primary_key.type))
                  for primary_key in self.primary_key]
        return sql.and_(*[k == v for (k, v) in params]), \
                util.column_dict(params)

    @_memoized_configured_property
    def _equivalent_columns(self):
        """Create a map of all *equivalent* columns, based on
        the determination of column pairs that are equated to
        one another based on inherit condition.  This is designed
        to work with the queries that util.polymorphic_union
        comes up with, which often don't include the columns from
        the base table directly (including the subclass table columns
        only).

        The resulting structure is a dictionary of columns mapped
        to lists of equivalent columns, i.e.

        {
            tablea.col1:
                set([tableb.col1, tablec.col1]),
            tablea.col2:
                set([tabled.col2])
        }

        """
        result = util.column_dict()

        def visit_binary(binary):
            if binary.operator == operators.eq:
                if binary.left in result:
                    result[binary.left].add(binary.right)
                else:
                    result[binary.left] = util.column_set((binary.right,))
                if binary.right in result:
                    result[binary.right].add(binary.left)
                else:
                    result[binary.right] = util.column_set((binary.left,))
        for mapper in self.base_mapper.self_and_descendants:
            if mapper.inherit_condition is not None:
                visitors.traverse(
                                    mapper.inherit_condition, {},
                                    {'binary': visit_binary})

        return result

    def _is_userland_descriptor(self, obj):
        if isinstance(obj, (_MappedAttribute,
                            instrumentation.ClassManager,
                            expression.ColumnElement)):
            return False
        else:
            return True

    def _should_exclude(self, name, assigned_name, local, column):
        """determine whether a particular property should be implicitly
        present on the class.

        This occurs when properties are propagated from an inherited class, or
        are applied from the columns present in the mapped table.

        """

        # check for class-bound attributes and/or descriptors,
        # either local or from an inherited class
        if local:
            if self.class_.__dict__.get(assigned_name, None) is not None \
                and self._is_userland_descriptor(
                            self.class_.__dict__[assigned_name]):
                return True
        else:
            if getattr(self.class_, assigned_name, None) is not None \
                and self._is_userland_descriptor(
                            getattr(self.class_, assigned_name)):
                return True

        if self.include_properties is not None and \
                name not in self.include_properties and \
                (column is None or column not in self.include_properties):
            self._log("not including property %s" % (name))
            return True

        if self.exclude_properties is not None and \
            (
                name in self.exclude_properties or \
                (column is not None and column in self.exclude_properties)
            ):
            self._log("excluding property %s" % (name))
            return True

        return False

    def common_parent(self, other):
        """Return true if the given mapper shares a
        common inherited parent as this mapper."""

        return self.base_mapper is other.base_mapper

    def _canload(self, state, allow_subtypes):
        s = self.primary_mapper()
        if self.polymorphic_on is not None or allow_subtypes:
            return _state_mapper(state).isa(s)
        else:
            return _state_mapper(state) is s

    def isa(self, other):
        """Return True if the this mapper inherits from the given mapper."""

        m = self
        while m and m is not other:
            m = m.inherits
        return bool(m)

    def iterate_to_root(self):
        m = self
        while m:
            yield m
            m = m.inherits

    @_memoized_configured_property
    def self_and_descendants(self):
        """The collection including this mapper and all descendant mappers.

        This includes not just the immediately inheriting mappers but
        all their inheriting mappers as well.

        """
        descendants = []
        stack = deque([self])
        while stack:
            item = stack.popleft()
            descendants.append(item)
            stack.extend(item._inheriting_mappers)
        return util.WeakSequence(descendants)

    def polymorphic_iterator(self):
        """Iterate through the collection including this mapper and
        all descendant mappers.

        This includes not just the immediately inheriting mappers but
        all their inheriting mappers as well.

        To iterate through an entire hierarchy, use
        ``mapper.base_mapper.polymorphic_iterator()``.

        """
        return iter(self.self_and_descendants)

    def primary_mapper(self):
        """Return the primary mapper corresponding to this mapper's class key
        (class)."""

        return self.class_manager.mapper

    @property
    def primary_base_mapper(self):
        return self.class_manager.mapper.base_mapper

    def identity_key_from_row(self, row, adapter=None):
        """Return an identity-map key for use in storing/retrieving an
        item from the identity map.

        row
          A ``sqlalchemy.engine.RowProxy`` instance or a
          dictionary corresponding result-set ``ColumnElement``
          instances to their values within a row.

        """
        pk_cols = self.primary_key
        if adapter:
            pk_cols = [adapter.columns[c] for c in pk_cols]

        return self._identity_class, \
                tuple(row[column] for column in pk_cols)

    def identity_key_from_primary_key(self, primary_key):
        """Return an identity-map key for use in storing/retrieving an
        item from an identity map.

        primary_key
          A list of values indicating the identifier.

        """
        return self._identity_class, tuple(primary_key)

    def identity_key_from_instance(self, instance):
        """Return the identity key for the given instance, based on
        its primary key attributes.

        This value is typically also found on the instance state under the
        attribute name `key`.

        """
        return self.identity_key_from_primary_key(
                        self.primary_key_from_instance(instance))

    def _identity_key_from_state(self, state):
        dict_ = state.dict
        manager = state.manager
        return self._identity_class, tuple([
            manager[self._columntoproperty[col].key].\
                impl.get(state, dict_, attributes.PASSIVE_OFF)
            for col in self.primary_key
        ])

    def primary_key_from_instance(self, instance):
        """Return the list of primary key values for the given
        instance.

        """
        state = attributes.instance_state(instance)
        return self._primary_key_from_state(state)

    def _primary_key_from_state(self, state):
        dict_ = state.dict
        manager = state.manager
        return [
            manager[self._columntoproperty[col].key].\
                impl.get(state, dict_, attributes.PASSIVE_OFF)
            for col in self.primary_key
        ]

    def _get_state_attr_by_column(self, state, dict_, column,
                                    passive=attributes.PASSIVE_OFF):
        prop = self._columntoproperty[column]
        return state.manager[prop.key].impl.get(state, dict_, passive=passive)

    def _set_state_attr_by_column(self, state, dict_, column, value):
        prop = self._columntoproperty[column]
        state.manager[prop.key].impl.set(state, dict_, value, None)

    def _get_committed_attr_by_column(self, obj, column):
        state = attributes.instance_state(obj)
        dict_ = attributes.instance_dict(obj)
        return self._get_committed_state_attr_by_column(state, dict_, column)

    def _get_committed_state_attr_by_column(self, state, dict_,
                        column, passive=attributes.PASSIVE_OFF):

        prop = self._columntoproperty[column]
        return state.manager[prop.key].impl.\
                    get_committed_value(state, dict_, passive=passive)

    def _optimized_get_statement(self, state, attribute_names):
        """assemble a WHERE clause which retrieves a given state by primary
        key, using a minimized set of tables.

        Applies to a joined-table inheritance mapper where the
        requested attribute names are only present on joined tables,
        not the base table.  The WHERE clause attempts to include
        only those tables to minimize joins.

        """
        props = self._props

        tables = set(chain(
                        *[sql_util.find_tables(c, check_columns=True)
                        for key in attribute_names
                        for c in props[key].columns]
                    ))

        if self.base_mapper.local_table in tables:
            return None

        class ColumnsNotAvailable(Exception):
            pass

        def visit_binary(binary):
            leftcol = binary.left
            rightcol = binary.right
            if leftcol is None or rightcol is None:
                return

            if leftcol.table not in tables:
                leftval = self._get_committed_state_attr_by_column(
                                    state, state.dict,
                                    leftcol,
                                    passive=attributes.PASSIVE_NO_INITIALIZE)
                if leftval is attributes.PASSIVE_NO_RESULT or leftval is None:
                    raise ColumnsNotAvailable()
                binary.left = sql.bindparam(None, leftval,
                                            type_=binary.right.type)
            elif rightcol.table not in tables:
                rightval = self._get_committed_state_attr_by_column(
                                    state, state.dict,
                                    rightcol,
                                    passive=attributes.PASSIVE_NO_INITIALIZE)
                if rightval is attributes.PASSIVE_NO_RESULT or \
                            rightval is None:
                    raise ColumnsNotAvailable()
                binary.right = sql.bindparam(None, rightval,
                                            type_=binary.right.type)

        allconds = []

        try:
            start = False
            for mapper in reversed(list(self.iterate_to_root())):
                if mapper.local_table in tables:
                    start = True
                elif not isinstance(mapper.local_table, expression.TableClause):
                    return None
                if start and not mapper.single:
                    allconds.append(visitors.cloned_traverse(
                                                mapper.inherit_condition,
                                                {},
                                                {'binary': visit_binary}
                                        )
                                    )
        except ColumnsNotAvailable:
            return None

        cond = sql.and_(*allconds)

        cols = []
        for key in attribute_names:
            cols.extend(props[key].columns)
        return sql.select(cols, cond, use_labels=True)

    def cascade_iterator(self, type_, state, halt_on=None):
        """Iterate each element and its mapper in an object graph,
        for all relationships that meet the given cascade rule.

        :param type_:
          The name of the cascade rule (i.e. save-update, delete,
          etc.)

        :param state:
          The lead InstanceState.  child items will be processed per
          the relationships defined for this object's mapper.

        the return value are object instances; this provides a strong
        reference so that they don't fall out of scope immediately.

        """
        visited_states = set()
        prp, mpp = object(), object()

        visitables = deque([(deque(self._props.values()), prp,
                                state, state.dict)])

        while visitables:
            iterator, item_type, parent_state, parent_dict = visitables[-1]
            if not iterator:
                visitables.pop()
                continue

            if item_type is prp:
                prop = iterator.popleft()
                if type_ not in prop.cascade:
                    continue
                queue = deque(prop.cascade_iterator(type_, parent_state,
                            parent_dict, visited_states, halt_on))
                if queue:
                    visitables.append((queue, mpp, None, None))
            elif item_type is mpp:
                instance, instance_mapper, corresponding_state, \
                                corresponding_dict = iterator.popleft()
                yield instance, instance_mapper, \
                        corresponding_state, corresponding_dict
                visitables.append((deque(instance_mapper._props.values()),
                                        prp, corresponding_state,
                                        corresponding_dict))

    @_memoized_configured_property
    def _compiled_cache(self):
        return util.LRUCache(self._compiled_cache_size)

    @_memoized_configured_property
    def _sorted_tables(self):
        table_to_mapper = {}

        for mapper in self.base_mapper.self_and_descendants:
            for t in mapper.tables:
                table_to_mapper.setdefault(t, mapper)

        extra_dependencies = []
        for table, mapper in table_to_mapper.items():
            super_ = mapper.inherits
            if super_:
                extra_dependencies.extend([
                    (super_table, table)
                    for super_table in super_.tables
                    ])

        def skip(fk):
            # attempt to skip dependencies that are not
            # significant to the inheritance chain
            # for two tables that are related by inheritance.
            # while that dependency may be important, it's techinically
            # not what we mean to sort on here.
            parent = table_to_mapper.get(fk.parent.table)
            dep = table_to_mapper.get(fk.column.table)
            if parent is not None and \
                dep is not None and \
                dep is not parent and \
                    dep.inherit_condition is not None:
                cols = set(sql_util._find_columns(dep.inherit_condition))
                if parent.inherit_condition is not None:
                    cols = cols.union(sql_util._find_columns(
                                        parent.inherit_condition))
                    return fk.parent not in cols and fk.column not in cols
                else:
                    return fk.parent not in cols
            return False

        sorted_ = sql_util.sort_tables(table_to_mapper,
                                    skip_fn=skip,
                                    extra_dependencies=extra_dependencies)

        ret = util.OrderedDict()
        for t in sorted_:
            ret[t] = table_to_mapper[t]
        return ret

    def _memo(self, key, callable_):
        if key in self._memoized_values:
            return self._memoized_values[key]
        else:
            self._memoized_values[key] = value = callable_()
            return value

    @util.memoized_property
    def _table_to_equated(self):
        """memoized map of tables to collections of columns to be
        synchronized upwards to the base mapper."""

        result = util.defaultdict(list)

        for table in self._sorted_tables:
            cols = set(table.c)
            for m in self.iterate_to_root():
                if m._inherits_equated_pairs and \
                    cols.intersection(
                        [l for l, r in m._inherits_equated_pairs]):
                    result[table].append((m, m._inherits_equated_pairs))

        return result


def configure_mappers():
    """Initialize the inter-mapper relationships of all mappers that
    have been constructed thus far.

    This function can be called any number of times, but in
    most cases is handled internally.

    """

    if not Mapper._new_mappers:
        return

    _call_configured = None
    _CONFIGURE_MUTEX.acquire()
    try:
        global _already_compiling
        if _already_compiling:
            return
        _already_compiling = True
        try:

            # double-check inside mutex
            if not Mapper._new_mappers:
                return

            # initialize properties on all mappers
            # note that _mapper_registry is unordered, which
            # may randomly conceal/reveal issues related to
            # the order of mapper compilation
            for mapper in list(_mapper_registry):
                if getattr(mapper, '_configure_failed', False):
                    e = sa_exc.InvalidRequestError(
                            "One or more mappers failed to initialize - "
                            "can't proceed with initialization of other "
                            "mappers.  Original exception was: %s"
                            % mapper._configure_failed)
                    e._configure_failed = mapper._configure_failed
                    raise e
                if not mapper.configured:
                    try:
                        mapper._post_configure_properties()
                        mapper._expire_memoizations()
                        mapper.dispatch.mapper_configured(
                                mapper, mapper.class_)
                        _call_configured = mapper
                    except:
                        exc = sys.exc_info()[1]
                        if not hasattr(exc, '_configure_failed'):
                            mapper._configure_failed = exc
                        raise

            Mapper._new_mappers = False
        finally:
            _already_compiling = False
    finally:
        _CONFIGURE_MUTEX.release()
    if _call_configured is not None:
        _call_configured.dispatch.after_configured()


def reconstructor(fn):
    """Decorate a method as the 'reconstructor' hook.

    Designates a method as the "reconstructor", an ``__init__``-like
    method that will be called by the ORM after the instance has been
    loaded from the database or otherwise reconstituted.

    The reconstructor will be invoked with no arguments.  Scalar
    (non-collection) database-mapped attributes of the instance will
    be available for use within the function.  Eagerly-loaded
    collections are generally not yet available and will usually only
    contain the first element.  ORM state changes made to objects at
    this stage will not be recorded for the next flush() operation, so
    the activity within a reconstructor should be conservative.

    """
    fn.__sa_reconstructor__ = True
    return fn


def validates(*names, **kw):
    """Decorate a method as a 'validator' for one or more named properties.

    Designates a method as a validator, a method which receives the
    name of the attribute as well as a value to be assigned, or in the
    case of a collection, the value to be added to the collection.
    The function can then raise validation exceptions to halt the
    process from continuing (where Python's built-in ``ValueError``
    and ``AssertionError`` exceptions are reasonable choices), or can
    modify or replace the value before proceeding. The function should
    otherwise return the given value.

    Note that a validator for a collection **cannot** issue a load of that
    collection within the validation routine - this usage raises
    an assertion to avoid recursion overflows.  This is a reentrant
    condition which is not supported.

    :param \*names: list of attribute names to be validated.
    :param include_removes: if True, "remove" events will be
     sent as well - the validation function must accept an additional
     argument "is_remove" which will be a boolean.

     .. versionadded:: 0.7.7

    """
    include_removes = kw.pop('include_removes', False)

    def wrap(fn):
        fn.__sa_validators__ = names
        fn.__sa_include_removes__ = include_removes
        return fn
    return wrap


def _event_on_load(state, ctx):
    instrumenting_mapper = state.manager.info[_INSTRUMENTOR]
    if instrumenting_mapper._reconstructor:
        instrumenting_mapper._reconstructor(state.obj())


def _event_on_first_init(manager, cls):
    """Initial mapper compilation trigger.

    instrumentation calls this one when InstanceState
    is first generated, and is needed for legacy mutable
    attributes to work.
    """

    instrumenting_mapper = manager.info.get(_INSTRUMENTOR)
    if instrumenting_mapper:
        if Mapper._new_mappers:
            configure_mappers()


def _event_on_init(state, args, kwargs):
    """Run init_instance hooks.

    This also includes mapper compilation, normally not needed
    here but helps with some piecemeal configuration
    scenarios (such as in the ORM tutorial).

    """

    instrumenting_mapper = state.manager.info.get(_INSTRUMENTOR)
    if instrumenting_mapper:
        if Mapper._new_mappers:
            configure_mappers()
        if instrumenting_mapper._set_polymorphic_identity:
            instrumenting_mapper._set_polymorphic_identity(state)


def _event_on_resurrect(state):
    # re-populate the primary key elements
    # of the dict based on the mapping.
    instrumenting_mapper = state.manager.info.get(_INSTRUMENTOR)
    if instrumenting_mapper:
        for col, val in zip(instrumenting_mapper.primary_key, state.key[1]):
            instrumenting_mapper._set_state_attr_by_column(
                                            state, state.dict, col, val)


class _ColumnMapping(dict):
    """Error reporting helper for mapper._columntoproperty."""

    def __init__(self, mapper):
        self.mapper = mapper

    def __missing__(self, column):
        prop = self.mapper._props.get(column)
        if prop:
            raise orm_exc.UnmappedColumnError(
                "Column '%s.%s' is not available, due to "
                "conflicting property '%s':%r" % (
                    column.table.name, column.name, column.key, prop))
        raise orm_exc.UnmappedColumnError(
            "No column %s is configured on mapper %s..." %
            (column, self.mapper))
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