Intermediate `__abstract__` inheriting from model doesn't transfer model's attrs

[David Lord]

I have the following models:

• Device
• DeviceSource has a foreign key and relationship to Device
• LDAPDeviceSource is a joined-table inheritance of DeviceSource

In order to make defining new DeviceSource subclasses easier, I created DeviceSourceMixin, which inherits from DeviceSource and provides declared attrs for __tablename__ and the foreign primary id. DeviceSourceMixin is __abstract__ so that it doesn't create a table of its own.

The issue is that this intermediate __abstract__ seems to break the declarative model. The foreign key and relationship in DeviceSource do not get inherited by LDAPDeviceSource when subclassing DeviceSourceMixin.

This code demonstrates the issue:

import sqlalchemy as sa
from sqlalchemy.ext.declarative import declared_attr, as_declarative
from sqlalchemy.orm import Session, relationship

engine = sa.create_engine('sqlite:///:memory:', echo=True)
session = Session(bind=engine)


@as_declarative(bind=engine)
class Base(object):
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

    id = sa.Column(sa.Integer, primary_key=True)


class Device(Base):
    pass


class DeviceSource(Base):
    type = sa.Column(sa.String, nullable=False)
    device_id = sa.Column(sa.Integer, sa.ForeignKey(Device.id), nullable=False)

    device = relationship(Device, backref='sources')

    __mapper_args__ = {
        'polymorphic_on': type
    }


class DeviceSourceMixin(DeviceSource):
    __abstract__ = True

    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

    @declared_attr
    def id(cls):
        return sa.Column(sa.Integer, sa.ForeignKey(DeviceSource.id), primary_key=True)


class LDAPDeviceSource(DeviceSourceMixin):
    name = sa.Column(sa.String, nullable=False)

    __mapper_args__ = {
        'polymorphic_identity': 'ldap'
    }


Base.metadata.create_all()
d1 = Device()
s1 = LDAPDeviceSource(device=d1, name='s1')
session.add(s1)
session.commit()

It produces the following error:

Traceback (most recent call last):
  File "/home/david/Projects/cedar/example2.py", line 62, in <module>
    s1 = LDAPDeviceSource(device=d1, name='s1')
  File "<string>", line 4, in __init__
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/state.py", line 260, in _initialize_instance
    return manager.original_init(*mixed[1:], **kwargs)
  File "<string>", line 6, in __init__
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/ext/declarative/base.py", line 526, in _declarative_constructor
    setattr(self, k, kwargs[k])
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 226, in __set__
    instance_dict(instance), value, None)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 812, in set
    value = self.fire_replace_event(state, dict_, value, old, initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 832, in fire_replace_event
    state, value, previous, initiator or self._replace_token)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 1148, in emit_backref_from_scalar_set_event
    passive=PASSIVE_NO_FETCH)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 980, in append
    collection.append_with_event(value, initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/collections.py", line 653, in append_with_event
    self._data()._sa_appender(item, _sa_initiator=initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/collections.py", line 1047, in append
    item = __set(self, item, _sa_initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/collections.py", line 1019, in __set
    item = executor.fire_append_event(item, _sa_initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/collections.py", line 716, in fire_append_event
    item, initiator)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 929, in fire_append_event
    value = fn(state, value, initiator or self._append_token)
  File "/home/david/.virtualenvs/cedar/lib/python3.4/site-packages/sqlalchemy/orm/attributes.py", line 1157, in emit_backref_from_collection_append_event
    child_impl = child_state.manager[key].impl
KeyError: 'device'

[David Lord]

I can "solve" it by making DeviceSourceMixin inherit from object, and just subclass both DeviceSourceMixin and DeviceSource, but it seems like this is something __abstract__ should be able to handle.

[Mike Bayer]

that would be the workaround for now, yes. intricate declarative fixes like this are targeted at 1.0 for now.

[Mike Bayer]

here's a patch

diff --git a/lib/sqlalchemy/ext/declarative/base.py b/lib/sqlalchemy/ext/declarative/base.py
index 291608b..451457a 100644
--- a/lib/sqlalchemy/ext/declarative/base.py
+++ b/lib/sqlalchemy/ext/declarative/base.py
@@ -35,6 +35,21 @@ def _declared_mapping_info(cls):
         return None


+def _resolve_for_abstract(cls):
+    if cls is object:
+        return None
+
+    if _get_immediate_cls_attr(cls, '__abstract__'):
+        for sup in cls.__bases__:
+            sup = _resolve_for_abstract(sup)
+            if sup is not None:
+                return sup
+        else:
+            return None
+    else:
+        return cls
+
+
 def _get_immediate_cls_attr(cls, attrname):
     """return an attribute of the class that is either present directly
     on the class, e.g. not on a superclass, or is from a superclass but
@@ -46,6 +61,9 @@ def _get_immediate_cls_attr(cls, attrname):
     inherit from.

     """
+    if not issubclass(cls, object):
+        return None
+
     for base in cls.__mro__:
         _is_declarative_inherits = hasattr(base, '_decl_class_registry')
         if attrname in base.__dict__:
@@ -388,6 +406,9 @@ class _MapperConfig(object):
         table_args = self.table_args
         declared_columns = self.declared_columns
         for c in cls.__bases__:
+            c = _resolve_for_abstract(c)
+            if c is None:
+                continue
             if _declared_mapping_info(c) is not None and \
                     not _get_immediate_cls_attr(
                         c, '_sa_decl_prepare_nocascade'):

[Mike Bayer]

Issue #3240 was marked as a duplicate of this issue.

[Mike Bayer]

make sure to add tests for the #3240 condition.

[Mike Bayer]

• Fixed bug where using an __abstract__ mixin in the middle of a declarative inheritance hierarchy would prevent attributes and configuration being correctly propagated from the base class to the inheriting class. fixes #3219 fixes #3240

→ <<cset 95e53d0b6072>>