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Using the Session

The :func:`.orm.mapper` function and :mod:`~sqlalchemy.ext.declarative` extensions are the primary configurational interface for the ORM. Once mappings are configured, the primary usage interface for persistence operations is the :class:`.Session`.

What does the Session do ?

In the most general sense, the :class:`~.Session` establishes all conversations with the database and represents a "holding zone" for all the objects which you've loaded or associated with it during its lifespan. It provides the entrypoint to acquire a :class:`.Query` object, which sends queries to the database using the :class:`~.Session` object's current database connection, populating result rows into objects that are then stored in the :class:`.Session`, inside a structure called the Identity Map - a data structure that maintains unique copies of each object, where "unique" means "only one object with a particular primary key".

The :class:`.Session` begins in an essentially stateless form. Once queries are issued or other objects are persisted with it, it requests a connection resource from an :class:`.Engine` that is associated either with the :class:`.Session` itself or with the mapped :class:`.Table` objects being operated upon. This connection represents an ongoing transaction, which remains in effect until the :class:`.Session` is instructed to commit or roll back its pending state.

All changes to objects maintained by a :class:`.Session` are tracked - before the database is queried again or before the current transaction is committed, it flushes all pending changes to the database. This is known as the Unit of Work pattern.

When using a :class:`.Session`, it's important to note that the objects which are associated with it are proxy objects to the transaction being held by the :class:`.Session` - there are a variety of events that will cause objects to re-access the database in order to keep synchronized. It is possible to "detach" objects from a :class:`.Session`, and to continue using them, though this practice has its caveats. It's intended that usually, you'd re-associate detached objects another :class:`.Session` when you want to work with them again, so that they can resume their normal task of representing database state.

Getting a Session

:class:`.Session` is a regular Python class which can be directly instantiated. However, to standardize how sessions are configured and acquired, the :class:`.sessionmaker` class is normally used to create a top level :class:`.Session` configuration which can then be used throughout an application without the need to repeat the configurational arguments.

The usage of :class:`.sessionmaker` is illustrated below:

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

# an Engine, which the Session will use for connection
# resources
some_engine = create_engine('postgresql://scott:tiger@localhost/')

# create a configured "Session" class
Session = sessionmaker(bind=some_engine)

# create a Session
session = Session()

# work with sess
myobject = MyObject('foo', 'bar')
session.add(myobject)
session.commit()

Above, the :class:`.sessionmaker` call creates a factory for us, which we assign to the name Session. This factory, when called, will create a new :class:`.Session` object using the configurational arguments we've given the factory. In this case, as is typical, we've configured the factory to specify a particular :class:`.Engine` for connection resources.

A typical setup will associate the :class:`.sessionmaker` with an :class:`.Engine`, so that each :class:`.Session` generated will use this :class:`.Engine` to acquire connection resources. This association can be set up as in the example above, using the bind argument.

When you write your application, place the :class:`.sessionmaker` factory at the global level. This factory can then be used by the rest of the applcation as the source of new :class:`.Session` instances, keeping the configuration for how :class:`.Session` objects are constructed in one place.

The :class:`.sessionmaker` factory can also be used in conjunction with other helpers, which are passed a user-defined :class:`.sessionmaker` that is then maintained by the helper. Some of these helpers are discussed in the section :ref:`session_faq_whentocreate`.

Adding Additional Configuration to an Existing sessionmaker()

A common scenario is where the :class:`.sessionmaker` is invoked at module import time, however the generation of one or more :class:`.Engine` instances to be associated with the :class:`.sessionmaker` has not yet proceeded. For this use case, the :class:`.sessionmaker` construct offers the :meth:`.sessionmaker.configure` method, which will place additional configuration directives into an existing :class:`.sessionmaker` that will take place when the construct is invoked:

from sqlalchemy.orm import sessionmaker
from sqlalchemy import create_engine

# configure Session class with desired options
Session = sessionmaker()

# later, we create the engine
engine = create_engine('postgresql://...')

# associate it with our custom Session class
Session.configure(bind=engine)

# work with the session
session = Session()

Creating Ad-Hoc Session Objects with Alternate Arguments

For the use case where an application needs to create a new :class:`.Session` with special arguments that deviate from what is normally used throughout the application, such as a :class:`.Session` that binds to an alternate source of connectivity, or a :class:`.Session` that should have other arguments such as expire_on_commit established differently from what most of the application wants, specific arguments can be passed to the :class:`.sessionmaker` factory's :meth:`.sessionmaker.__call__` method. These arguments will override whatever configurations have already been placed, such as below, where a new :class:`.Session` is constructed against a specific :class:`.Connection`:

# at the module level, the global sessionmaker,
# bound to a specific Engine
Session = sessionmaker(bind=engine)

# later, some unit of code wants to create a
# Session that is bound to a specific Connection
conn = engine.connect()
session = Session(bind=conn)

The typical rationale for the association of a :class:`.Session` with a specific :class:`.Connection` is that of a test fixture that maintains an external transaction - see :ref:`session_external_transaction` for an example of this.

Using the Session

Quickie Intro to Object States

It's helpful to know the states which an instance can have within a session:

  • Transient - an instance that's not in a session, and is not saved to the database; i.e. it has no database identity. The only relationship such an object has to the ORM is that its class has a mapper() associated with it.
  • Pending - when you :func:`~sqlalchemy.orm.session.Session.add` a transient instance, it becomes pending. It still wasn't actually flushed to the database yet, but it will be when the next flush occurs.
  • Persistent - An instance which is present in the session and has a record in the database. You get persistent instances by either flushing so that the pending instances become persistent, or by querying the database for existing instances (or moving persistent instances from other sessions into your local session).
  • Detached - an instance which has a record in the database, but is not in any session. There's nothing wrong with this, and you can use objects normally when they're detached, except they will not be able to issue any SQL in order to load collections or attributes which are not yet loaded, or were marked as "expired".

Knowing these states is important, since the :class:`.Session` tries to be strict about ambiguous operations (such as trying to save the same object to two different sessions at the same time).

Session Frequently Asked Questions

  • When do I make a :class:`.sessionmaker` ?

    Just one time, somewhere in your application's global scope. It should be looked upon as part of your application's configuration. If your application has three .py files in a package, you could, for example, place the :class:`.sessionmaker` line in your __init__.py file; from that point on your other modules say "from mypackage import Session". That way, everyone else just uses :class:`.Session()`, and the configuration of that session is controlled by that central point.

    If your application starts up, does imports, but does not know what database it's going to be connecting to, you can bind the :class:`.Session` at the "class" level to the engine later on, using :meth:`.sessionmaker.configure`.

    In the examples in this section, we will frequently show the :class:`.sessionmaker` being created right above the line where we actually invoke :class:`.Session`. But that's just for example's sake! In reality, the :class:`.sessionmaker` would be somewhere at the module level. The calls to instantiate :class:`.Session` would then be placed at the point in the application where database conversations begin.

  • When do I construct a :class:`.Session`, when do I commit it, and when do I close it ?

    A :class:`.Session` is typically constructed at the beginning of a logical operation where database access is potentially anticipated.

    The :class:`.Session`, whenever it is used to talk to the database, begins a database transaction as soon as it starts communicating. Assuming the autocommit flag is left at its recommended default of False, this transaction remains in progress until the :class:`.Session` is rolled back, committed, or closed. The :class:`.Session` will begin a new transaction if it is used again, subsequent to the previous transaction ending; from this it follows that the :class:`.Session` is capable of having a lifespan across many transactions, though only one at a time. We refer to these two concepts as transaction scope and session scope.

    The implication here is that the SQLAlchemy ORM is encouraging the developer to establish these two scopes in his or her application, including not only when the scopes begin and end, but also the expanse of those scopes, for example should a single :class:`.Session` instance be local to the execution flow within a function or method, should it be a global object used by the entire application, or somewhere in between these two.

    The burden placed on the developer to determine this scope is one area where the SQLAlchemy ORM necessarily has a strong opinion about how the database should be used. The unit-of-work pattern is specifically one of accumulating changes over time and flushing them periodically, keeping in-memory state in sync with what's known to be present in a local transaction. This pattern is only effective when meaningful transaction scopes are in place.

    It's usually not very hard to determine the best points at which to begin and end the scope of a :class:`.Session`, though the wide variety of application architectures possible can introduce challenging situations.

    A common choice is to tear down the :class:`.Session` at the same time the transaction ends, meaning the transaction and session scopes are the same. This is a great choice to start out with as it removes the need to consider session scope as separate from transaction scope.

    While there's no one-size-fits-all recommendation for how transaction scope should be determined, there are common patterns. Especially if one is writing a web application, the choice is pretty much established.

    A web application is the easiest case because such an appication is already constructed around a single, consistent scope - this is the request, which represents an incoming request from a browser, the processing of that request to formulate a response, and finally the delivery of that response back to the client. Integrating web applications with the :class:`.Session` is then the straightforward task of linking the scope of the :class:`.Session` to that of the request. The :class:`.Session` can be established as the request begins, or using a lazy initialization pattern which establishes one as soon as it is needed. The request then proceeds, with some system in place where application logic can access the current :class:`.Session` in a manner associated with how the actual request object is accessed. As the request ends, the :class:`.Session` is torn down as well, usually through the usage of event hooks provided by the web framework. The transaction used by the :class:`.Session` may also be committed at this point, or alternatively the application may opt for an explicit commit pattern, only committing for those requests where one is warranted, but still always tearing down the :class:`.Session` unconditionally at the end.

    Most web frameworks include infrastructure to establish a single :class:`.Session`, associated with the request, which is correctly constructed and torn down corresponding torn down at the end of a request. Such infrastructure pieces include products such as Flask-SQLAlchemy, for usage in conjunction with the Flask web framework, and Zope-SQLAlchemy, for usage in conjunction with the Pyramid and Zope frameworks. SQLAlchemy strongly recommends that these products be used as available.

    In those situations where integration libraries are not available, SQLAlchemy includes its own "helper" class known as :class:`.scoped_session`. A tutorial on the usage of this object is at :ref:`unitofwork_contextual`. It provides both a quick way to associate a :class:`.Session` with the current thread, as well as patterns to associate :class:`.Session` objects with other kinds of scopes.

    As mentioned before, for non-web applications there is no one clear pattern, as applications themselves don't have just one pattern of architecture. The best strategy is to attempt to demarcate "operations", points at which a particular thread begins to perform a series of operations for some period of time, which can be committed at the end. Some examples:

    • A background daemon which spawns off child forks would want to create a :class:`.Session` local to each child process work with that :class:`.Session` through the life of the "job" that the fork is handling, then tear it down when the job is completed.
    • For a command-line script, the application would create a single, global :class:`.Session` that is established when the program begins to do its work, and commits it right as the program is completing its task.
    • For a GUI interface-driven application, the scope of the :class:`.Session` may best be within the scope of a user-generated event, such as a button push. Or, the scope may correspond to explicit user interaction, such as the user "opening" a series of records, then "saving" them.
  • Is the Session a cache ?

    Yeee...no. It's somewhat used as a cache, in that it implements the identity map pattern, and stores objects keyed to their primary key. However, it doesn't do any kind of query caching. This means, if you say session.query(Foo).filter_by(name='bar'), even if Foo(name='bar') is right there, in the identity map, the session has no idea about that. It has to issue SQL to the database, get the rows back, and then when it sees the primary key in the row, then it can look in the local identity map and see that the object is already there. It's only when you say query.get({some primary key}) that the :class:`~sqlalchemy.orm.session.Session` doesn't have to issue a query.

    Additionally, the Session stores object instances using a weak reference by default. This also defeats the purpose of using the Session as a cache.

    The :class:`.Session` is not designed to be a global object from which everyone consults as a "registry" of objects. That's more the job of a second level cache. SQLAlchemy provides a pattern for implementing second level caching using Beaker, via the :ref:`examples_caching` example.

  • How can I get the :class:`~sqlalchemy.orm.session.Session` for a certain object ?

    Use the :func:`~sqlalchemy.orm.session.Session.object_session` classmethod available on :class:`~sqlalchemy.orm.session.Session`:

    session = Session.object_session(someobject)
    
  • Is the session thread-safe?

    The :class:`.Session` is very much intended to be used in a non-concurrent fashion, which usually means in only one thread at a time.

    Nope. It has no thread synchronization of any kind built in, and particularly when you do a flush operation, it definitely is not open to concurrent threads accessing it, because it holds onto a single database connection at that point. If you use a session which is non-transactional (meaning, autocommit is set to True, not the default setting) for read operations only, it's still not thread-"safe", but you also wont get any catastrophic failures either, since it checks out and returns connections to the connection pool on an as-needed basis; it's just that different threads might load the same objects independently of each other, but only one will wind up in the identity map (however, the other one might still live in a collection somewhere).

    But the bigger point here is, you should not want to use the session with multiple concurrent threads. That would be like having everyone at a restaurant all eat from the same plate. The session is a local "workspace" that you use for a specific set of tasks; you don't want to, or need to, share that session with other threads who are doing some other task. If, on the other hand, there are other threads participating in the same task you are, such as in a desktop graphical application, then you would be sharing the session with those threads, but you also will have implemented a proper locking scheme (or your graphical framework does) so that those threads do not collide.

    A multithreaded application is usually going to want to make usage of :class:`.scoped_session` to transparently manage sessions per thread. More on this at :ref:`unitofwork_contextual`.

Querying

The :func:`~sqlalchemy.orm.session.Session.query` function takes one or more entities and returns a new :class:`~sqlalchemy.orm.query.Query` object which will issue mapper queries within the context of this Session. An entity is defined as a mapped class, a :class:`~sqlalchemy.orm.mapper.Mapper` object, an orm-enabled descriptor, or an AliasedClass object:

# query from a class
session.query(User).filter_by(name='ed').all()

# query with multiple classes, returns tuples
session.query(User, Address).join('addresses').filter_by(name='ed').all()

# query using orm-enabled descriptors
session.query(User.name, User.fullname).all()

# query from a mapper
user_mapper = class_mapper(User)
session.query(user_mapper)

When :class:`~sqlalchemy.orm.query.Query` returns results, each object instantiated is stored within the identity map. When a row matches an object which is already present, the same object is returned. In the latter case, whether or not the row is populated onto an existing object depends upon whether the attributes of the instance have been expired or not. A default-configured :class:`~sqlalchemy.orm.session.Session` automatically expires all instances along transaction boundaries, so that with a normally isolated transaction, there shouldn't be any issue of instances representing data which is stale with regards to the current transaction.

The :class:`.Query` object is introduced in great detail in :ref:`ormtutorial_toplevel`, and further documented in :ref:`query_api_toplevel`.

Adding New or Existing Items

:func:`~sqlalchemy.orm.session.Session.add` is used to place instances in the session. For transient (i.e. brand new) instances, this will have the effect of an INSERT taking place for those instances upon the next flush. For instances which are persistent (i.e. were loaded by this session), they are already present and do not need to be added. Instances which are detached (i.e. have been removed from a session) may be re-associated with a session using this method:

user1 = User(name='user1')
user2 = User(name='user2')
session.add(user1)
session.add(user2)

session.commit()     # write changes to the database

To add a list of items to the session at once, use :func:`~sqlalchemy.orm.session.Session.add_all`:

session.add_all([item1, item2, item3])

The :func:`~sqlalchemy.orm.session.Session.add` operation cascades along the save-update cascade. For more details see the section :ref:`unitofwork_cascades`.

Merging

:func:`~sqlalchemy.orm.session.Session.merge` transfers state from an outside object into a new or already existing instance within a session. It also reconciles the incoming data against the state of the database, producing a history stream which will be applied towards the next flush, or alternatively can be made to produce a simple "transfer" of state without producing change history or accessing the database. Usage is as follows:

merged_object = session.merge(existing_object)

When given an instance, it follows these steps:

  • It examines the primary key of the instance. If it's present, it attempts to locate that instance in the local identity map. If the load=True flag is left at its default, it also checks the database for this primary key if not located locally.

  • If the given instance has no primary key, or if no instance can be found with the primary key given, a new instance is created.

  • The state of the given instance is then copied onto the located/newly created instance. For attributes which are present on the source instance, the value is transferred to the target instance. For mapped attributes which aren't present on the source, the attribute is expired on the target instance, discarding its existing value.

    If the load=True flag is left at its default, this copy process emits events and will load the target object's unloaded collections for each attribute present on the source object, so that the incoming state can be reconciled against what's present in the database. If load is passed as False, the incoming data is "stamped" directly without producing any history.

  • The operation is cascaded to related objects and collections, as indicated by the merge cascade (see :ref:`unitofwork_cascades`).

  • The new instance is returned.

With :meth:`~.Session.merge`, the given "source" instance is not modifed nor is it associated with the target :class:`.Session`, and remains available to be merged with any number of other :class:`.Session` objects. :meth:`~.Session.merge` is useful for taking the state of any kind of object structure without regard for its origins or current session associations and copying its state into a new session. Here's some examples:

  • An application which reads an object structure from a file and wishes to save it to the database might parse the file, build up the structure, and then use :meth:`~.Session.merge` to save it to the database, ensuring that the data within the file is used to formulate the primary key of each element of the structure. Later, when the file has changed, the same process can be re-run, producing a slightly different object structure, which can then be merged in again, and the :class:`~sqlalchemy.orm.session.Session` will automatically update the database to reflect those changes, loading each object from the database by primary key and then updating its state with the new state given.

  • An application is storing objects in an in-memory cache, shared by many :class:`.Session` objects simultaneously. :meth:`~.Session.merge` is used each time an object is retrieved from the cache to create a local copy of it in each :class:`.Session` which requests it. The cached object remains detached; only its state is moved into copies of itself that are local to individual :class:`~.Session` objects.

    In the caching use case, it's common that the load=False flag is used to remove the overhead of reconciling the object's state with the database. There's also a "bulk" version of :meth:`~.Session.merge` called :meth:`~.Query.merge_result` that was designed to work with cache-extended :class:`.Query` objects - see the section :ref:`examples_caching`.

  • An application wants to transfer the state of a series of objects into a :class:`.Session` maintained by a worker thread or other concurrent system. :meth:`~.Session.merge` makes a copy of each object to be placed into this new :class:`.Session`. At the end of the operation, the parent thread/process maintains the objects it started with, and the thread/worker can proceed with local copies of those objects.

    In the "transfer between threads/processes" use case, the application may want to use the load=False flag as well to avoid overhead and redundant SQL queries as the data is transferred.

Merge Tips

:meth:`~.Session.merge` is an extremely useful method for many purposes. However, it deals with the intricate border between objects that are transient/detached and those that are persistent, as well as the automated transferrence of state. The wide variety of scenarios that can present themselves here often require a more careful approach to the state of objects. Common problems with merge usually involve some unexpected state regarding the object being passed to :meth:`~.Session.merge`.

Lets use the canonical example of the User and Address objects:

class User(Base):
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)
    name = Column(String(50), nullable=False)
    addresses = relationship("Address", backref="user")

class Address(Base):
    __tablename__ = 'address'

    id = Column(Integer, primary_key=True)
    email_address = Column(String(50), nullable=False)
    user_id = Column(Integer, ForeignKey('user.id'), nullable=False)

Assume a User object with one Address, already persistent:

>>> u1 = User(name='ed', addresses=[Address(email_address='ed@ed.com')])
>>> session.add(u1)
>>> session.commit()

We now create a1, an object outside the session, which we'd like to merge on top of the existing Address:

>>> existing_a1 = u1.addresses[0]
>>> a1 = Address(id=existing_a1.id)

A surprise would occur if we said this:

>>> a1.user = u1
>>> a1 = session.merge(a1)
>>> session.commit()
sqlalchemy.orm.exc.FlushError: New instance <Address at 0x1298f50>
with identity key (<class '__main__.Address'>, (1,)) conflicts with
persistent instance <Address at 0x12a25d0>

Why is that ? We weren't careful with our cascades. The assignment of a1.user to a persistent object cascaded to the backref of User.addresses and made our a1 object pending, as though we had added it. Now we have two Address objects in the session:

>>> a1 = Address()
>>> a1.user = u1
>>> a1 in session
True
>>> existing_a1 in session
True
>>> a1 is existing_a1
False

Above, our a1 is already pending in the session. The subsequent :meth:`~.Session.merge` operation essentially does nothing. Cascade can be configured via the cascade option on :func:`.relationship`, although in this case it would mean removing the save-update cascade from the User.addresses relationship - and usually, that behavior is extremely convenient. The solution here would usually be to not assign a1.user to an object already persistent in the target session.

The cascade_backrefs=False option of :func:`.relationship` will also prevent the Address from being added to the session via the a1.user = u1 assignment.

Further detail on cascade operation is at :ref:`unitofwork_cascades`.

Another example of unexpected state:

>>> a1 = Address(id=existing_a1.id, user_id=u1.id)
>>> assert a1.user is None
>>> True
>>> a1 = session.merge(a1)
>>> session.commit()
sqlalchemy.exc.IntegrityError: (IntegrityError) address.user_id
may not be NULL

Here, we accessed a1.user, which returned its default value of None, which as a result of this access, has been placed in the __dict__ of our object a1. Normally, this operation creates no change event, so the user_id attribute takes precedence during a flush. But when we merge the Address object into the session, the operation is equivalent to:

>>> existing_a1.id = existing_a1.id
>>> existing_a1.user_id = u1.id
>>> existing_a1.user = None

Where above, both user_id and user are assigned to, and change events are emitted for both. The user association takes precedence, and None is applied to user_id, causing a failure.

Most :meth:`~.Session.merge` issues can be examined by first checking - is the object prematurely in the session ?

>>> a1 = Address(id=existing_a1, user_id=user.id)
>>> assert a1 not in session
>>> a1 = session.merge(a1)

Or is there state on the object that we don't want ? Examining __dict__ is a quick way to check:

>>> a1 = Address(id=existing_a1, user_id=user.id)
>>> a1.user
>>> a1.__dict__
{'_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x1298d10>,
    'user_id': 1,
    'id': 1,
    'user': None}
>>> # we don't want user=None merged, remove it
>>> del a1.user
>>> a1 = session.merge(a1)
>>> # success
>>> session.commit()

Deleting

The :meth:`~.Session.delete` method places an instance into the Session's list of objects to be marked as deleted:

# mark two objects to be deleted
session.delete(obj1)
session.delete(obj2)

# commit (or flush)
session.commit()

Deleting from Collections

A common confusion that arises regarding :meth:`~.Session.delete` is when objects which are members of a collection are being deleted. While the collection member is marked for deletion from the database, this does not impact the collection itself in memory until the collection is expired. Below, we illustrate that even after an Address object is marked for deletion, it's still present in the collection associated with the parent User, even after a flush:

>>> address = user.addresses[1]
>>> session.delete(address)
>>> session.flush()
>>> address in user.addresses
True

When the above session is committed, all attributes are expired. The next access of user.addresses will re-load the collection, revealing the desired state:

>>> session.commit()
>>> address in user.addresses
False

The usual practice of deleting items within collections is to forego the usage of :meth:`~.Session.delete` directly, and instead use cascade behavior to automatically invoke the deletion as a result of removing the object from the parent collection. The delete-orphan cascade accomplishes this, as illustrated in the example below:

mapper(User, users_table, properties={
    'addresses':relationship(Address, cascade="all, delete, delete-orphan")
})
del user.addresses[1]
session.flush()

Where above, upon removing the Address object from the User.addresses collection, the delete-orphan cascade has the effect of marking the Address object for deletion in the same way as passing it to :meth:`~.Session.delete`.

See also :ref:`unitofwork_cascades` for detail on cascades.

Deleting based on Filter Criterion

The caveat with Session.delete() is that you need to have an object handy already in order to delete. The Query includes a :func:`~sqlalchemy.orm.query.Query.delete` method which deletes based on filtering criteria:

session.query(User).filter(User.id==7).delete()

The Query.delete() method includes functionality to "expire" objects already in the session which match the criteria. However it does have some caveats, including that "delete" and "delete-orphan" cascades won't be fully expressed for collections which are already loaded. See the API docs for :meth:`~sqlalchemy.orm.query.Query.delete` for more details.

Flushing

When the :class:`~sqlalchemy.orm.session.Session` is used with its default configuration, the flush step is nearly always done transparently. Specifically, the flush occurs before any individual :class:`~sqlalchemy.orm.query.Query` is issued, as well as within the :func:`~sqlalchemy.orm.session.Session.commit` call before the transaction is committed. It also occurs before a SAVEPOINT is issued when :func:`~sqlalchemy.orm.session.Session.begin_nested` is used.

Regardless of the autoflush setting, a flush can always be forced by issuing :func:`~sqlalchemy.orm.session.Session.flush`:

session.flush()

The "flush-on-Query" aspect of the behavior can be disabled by constructing :class:`.sessionmaker` with the flag autoflush=False:

Session = sessionmaker(autoflush=False)

Additionally, autoflush can be temporarily disabled by setting the autoflush flag at any time:

mysession = Session()
mysession.autoflush = False

Some autoflush-disable recipes are available at DisableAutoFlush.

The flush process always occurs within a transaction, even if the :class:`~sqlalchemy.orm.session.Session` has been configured with autocommit=True, a setting that disables the session's persistent transactional state. If no transaction is present, :func:`~sqlalchemy.orm.session.Session.flush` creates its own transaction and commits it. Any failures during flush will always result in a rollback of whatever transaction is present. If the Session is not in autocommit=True mode, an explicit call to :func:`~sqlalchemy.orm.session.Session.rollback` is required after a flush fails, even though the underlying transaction will have been rolled back already - this is so that the overall nesting pattern of so-called "subtransactions" is consistently maintained.

Committing

:func:`~sqlalchemy.orm.session.Session.commit` is used to commit the current transaction. It always issues :func:`~sqlalchemy.orm.session.Session.flush` beforehand to flush any remaining state to the database; this is independent of the "autoflush" setting. If no transaction is present, it raises an error. Note that the default behavior of the :class:`~sqlalchemy.orm.session.Session` is that a "transaction" is always present; this behavior can be disabled by setting autocommit=True. In autocommit mode, a transaction can be initiated by calling the :func:`~sqlalchemy.orm.session.Session.begin` method.

Note

The term "transaction" here refers to a transactional construct within the :class:`.Session` itself which may be maintaining zero or more actual database (DBAPI) transactions. An individual DBAPI connection begins participation in the "transaction" as it is first used to execute a SQL statement, then remains present until the session-level "transaction" is completed. See :ref:`unitofwork_transaction` for further detail.

Another behavior of :func:`~sqlalchemy.orm.session.Session.commit` is that by default it expires the state of all instances present after the commit is complete. This is so that when the instances are next accessed, either through attribute access or by them being present in a :class:`~sqlalchemy.orm.query.Query` result set, they receive the most recent state. To disable this behavior, configure :class:`.sessionmaker` with expire_on_commit=False.

Normally, instances loaded into the :class:`~sqlalchemy.orm.session.Session` are never changed by subsequent queries; the assumption is that the current transaction is isolated so the state most recently loaded is correct as long as the transaction continues. Setting autocommit=True works against this model to some degree since the :class:`~sqlalchemy.orm.session.Session` behaves in exactly the same way with regard to attribute state, except no transaction is present.

Rolling Back

:func:`~sqlalchemy.orm.session.Session.rollback` rolls back the current transaction. With a default configured session, the post-rollback state of the session is as follows:

  • All transactions are rolled back and all connections returned to the connection pool, unless the Session was bound directly to a Connection, in which case the connection is still maintained (but still rolled back).
  • Objects which were initially in the pending state when they were added to the :class:`~sqlalchemy.orm.session.Session` within the lifespan of the transaction are expunged, corresponding to their INSERT statement being rolled back. The state of their attributes remains unchanged.
  • Objects which were marked as deleted within the lifespan of the transaction are promoted back to the persistent state, corresponding to their DELETE statement being rolled back. Note that if those objects were first pending within the transaction, that operation takes precedence instead.
  • All objects not expunged are fully expired.

With that state understood, the :class:`~sqlalchemy.orm.session.Session` may safely continue usage after a rollback occurs.

When a :func:`~sqlalchemy.orm.session.Session.flush` fails, typically for reasons like primary key, foreign key, or "not nullable" constraint violations, a :func:`~sqlalchemy.orm.session.Session.rollback` is issued automatically (it's currently not possible for a flush to continue after a partial failure). However, the flush process always uses its own transactional demarcator called a subtransaction, which is described more fully in the docstrings for :class:`~sqlalchemy.orm.session.Session`. What it means here is that even though the database transaction has been rolled back, the end user must still issue :func:`~sqlalchemy.orm.session.Session.rollback` to fully reset the state of the :class:`~sqlalchemy.orm.session.Session`.

Expunging

Expunge removes an object from the Session, sending persistent instances to the detached state, and pending instances to the transient state:

session.expunge(obj1)

To remove all items, call :func:`~sqlalchemy.orm.session.Session.expunge_all` (this method was formerly known as clear()).

Closing

The :func:`~sqlalchemy.orm.session.Session.close` method issues a :func:`~sqlalchemy.orm.session.Session.expunge_all`, and releases any transactional/connection resources. When connections are returned to the connection pool, transactional state is rolled back as well.

Refreshing / Expiring

The Session normally works in the context of an ongoing transaction (with the default setting of autoflush=False). Most databases offer "isolated" transactions - this refers to a series of behaviors that allow the work within a transaction to remain consistent as time passes, regardless of the activities outside of that transaction. A key feature of a high degree of transaction isolation is that emitting the same SELECT statement twice will return the same results as when it was called the first time, even if the data has been modified in another transaction.

For this reason, the :class:`.Session` gains very efficient behavior by loading the attributes of each instance only once. Subsequent reads of the same row in the same transaction are assumed to have the same value. The user application also gains directly from this assumption, that the transaction is regarded as a temporary shield against concurrent changes - a good application will ensure that isolation levels are set appropriately such that this assumption can be made, given the kind of data being worked with.

To clear out the currently loaded state on an instance, the instance or its individual attributes can be marked as "expired", which results in a reload to occur upon next access of any of the instance's attrbutes. The instance can also be immediately reloaded from the database. The :meth:`~.Session.expire` and :meth:`~.Session.refresh` methods achieve this:

# immediately re-load attributes on obj1, obj2
session.refresh(obj1)
session.refresh(obj2)

# expire objects obj1, obj2, attributes will be reloaded
# on the next access:
session.expire(obj1)
session.expire(obj2)

When an expired object reloads, all non-deferred column-based attributes are loaded in one query. Current behavior for expired relationship-based attributes is that they load individually upon access - this behavior may be enhanced in a future release. When a refresh is invoked on an object, the ultimate operation is equivalent to a :meth:`.Query.get`, so any relationships configured with eager loading should also load within the scope of the refresh operation.

:meth:`~.Session.refresh` and :meth:`~.Session.expire` also support being passed a list of individual attribute names in which to be refreshed. These names can refer to any attribute, column-based or relationship based:

# immediately re-load the attributes 'hello', 'world' on obj1, obj2
session.refresh(obj1, ['hello', 'world'])
session.refresh(obj2, ['hello', 'world'])

# expire the attributes 'hello', 'world' objects obj1, obj2, attributes will be reloaded
# on the next access:
session.expire(obj1, ['hello', 'world'])
session.expire(obj2, ['hello', 'world'])

The full contents of the session may be expired at once using :meth:`~.Session.expire_all`:

session.expire_all()

Note that :meth:`~.Session.expire_all` is called automatically whenever :meth:`~.Session.commit` or :meth:`~.Session.rollback` are called. If using the session in its default mode of autocommit=False and with a well-isolated transactional environment (which is provided by most backends with the notable exception of MySQL MyISAM), there is virtually no reason to ever call :meth:`~.Session.expire_all` directly - plenty of state will remain on the current transaction until it is rolled back or committed or otherwise removed.

:meth:`~.Session.refresh` and :meth:`~.Session.expire` similarly are usually only necessary when an UPDATE or DELETE has been issued manually within the transaction using :meth:`.Session.execute()`.

Session Attributes

The :class:`~sqlalchemy.orm.session.Session` itself acts somewhat like a set-like collection. All items present may be accessed using the iterator interface:

for obj in session:
    print obj

And presence may be tested for using regular "contains" semantics:

if obj in session:
    print "Object is present"

The session is also keeping track of all newly created (i.e. pending) objects, all objects which have had changes since they were last loaded or saved (i.e. "dirty"), and everything that's been marked as deleted:

# pending objects recently added to the Session
session.new

# persistent objects which currently have changes detected
# (this collection is now created on the fly each time the property is called)
session.dirty

# persistent objects that have been marked as deleted via session.delete(obj)
session.deleted

# dictionary of all persistent objects, keyed on their
# identity key
session.identity_map

(Documentation: :attr:`.Session.new`, :attr:`.Session.dirty`, :attr:`.Session.deleted`, :attr:`.Session.identity_map`).

Note that objects within the session are by default weakly referenced. This means that when they are dereferenced in the outside application, they fall out of scope from within the :class:`~sqlalchemy.orm.session.Session` as well and are subject to garbage collection by the Python interpreter. The exceptions to this include objects which are pending, objects which are marked as deleted, or persistent objects which have pending changes on them. After a full flush, these collections are all empty, and all objects are again weakly referenced. To disable the weak referencing behavior and force all objects within the session to remain until explicitly expunged, configure :class:`.sessionmaker` with the weak_identity_map=False setting.

Cascades

Mappers support the concept of configurable cascade behavior on :func:`~sqlalchemy.orm.relationship` constructs. This refers to how operations performed on a parent object relative to a particular :class:`.Session` should be propagated to items referred to by that relationship. The default cascade behavior is usually suitable for most situations, and the option is normally invoked explicitly in order to enable delete and delete-orphan cascades, which refer to how the relationship should be treated when the parent is marked for deletion as well as when a child is de-associated from its parent.

Cascade behavior is configured by setting the cascade keyword argument on :func:`~sqlalchemy.orm.relationship`:

class Order(Base):
    __tablename__ = 'order'

    items = relationship("Item", cascade="all, delete-orphan")
    customer = relationship("User", secondary=user_orders_table,
                                cascade="save-update")

To set cascades on a backref, the same flag can be used with the :func:`~.sqlalchemy.orm.backref` function, which ultimately feeds its arguments back into :func:`~sqlalchemy.orm.relationship`:

class Item(Base):
    __tablename__ = 'item'

    order = relationship("Order",
                    backref=backref("items", cascade="all, delete-orphan")
                )

The default value of cascade is save-update, merge. The all symbol in the cascade options indicates that all cascade flags should be enabled, with the exception of delete-orphan. Typically, cascade is usually left at its default, or configured as all, delete-orphan, indicating the child objects should be treated as "owned" by the parent.

The list of available values which can be specified in cascade are as follows:

  • save-update - Indicates that when an object is placed into a :class:`.Session` via :meth:`.Session.add`, all the objects associated with it via this :func:`~sqlalchemy.orm.relationship` should also be added to that same :class:`.Session`. Additionally, if this object is already present in a :class:`.Session`, child objects will be added to that session as they are associated with this parent, i.e. as they are appended to lists, added to sets, or otherwise associated with the parent.

    save-update cascade also cascades the pending history of the target attribute, meaning that objects which were removed from a scalar or collection attribute whose changes have not yet been flushed are also placed into the target session. This is because they may have foreign key attributes present which will need to be updated to no longer refer to the parent.

    The save-update cascade is on by default, and it's common to not even be aware of it. It's customary that only a single call to :meth:`.Session.add` against the lead object of a structure has the effect of placing the full structure of objects into the :class:`.Session` at once.

    However, it can be turned off, which would imply that objects associated with a parent would need to be placed individually using :meth:`.Session.add` calls for each one.

    Another default behavior of save-update cascade is that it will take effect in the reverse direction, that is, associating a child with a parent when a backref is present means both relationships are affected; the parent will be added to the child's session. To disable this somewhat indirect session addition, use the cascade_backrefs=False option described below in :ref:`backref_cascade`.

  • delete - This cascade indicates that when the parent object is marked for deletion, the related objects should also be marked for deletion. Without this cascade present, SQLAlchemy will set the foreign key on a one-to-many relationship to NULL when the parent object is deleted. When enabled, the row is instead deleted.

    delete cascade is often used in conjunction with delete-orphan cascade, as is appropriate for an object whose foreign key is not intended to be nullable. On some backends, it's also a good idea to set ON DELETE on the foreign key itself; see the section :ref:`passive_deletes` for more details.

    Note that for many-to-many relationships which make usage of the secondary argument to :func:`~.sqlalchemy.orm.relationship`, SQLAlchemy always emits a DELETE for the association row in between "parent" and "child", when the parent is deleted or whenever the linkage between a particular parent and child is broken.

  • delete-orphan - This cascade adds behavior to the delete cascade, such that a child object will be marked for deletion when it is de-associated from the parent, not just when the parent is marked for deletion. This is a common feature when dealing with a related object that is "owned" by its parent, with a NOT NULL foreign key, so that removal of the item from the parent collection results in its deletion.

    delete-orphan cascade implies that each child object can only have one parent at a time, so is configured in the vast majority of cases on a one-to-many relationship. Setting it on a many-to-one or many-to-many relationship is more awkward; for this use case, SQLAlchemy requires that the :func:`~sqlalchemy.orm.relationship` be configured with the single_parent=True function, which establishes Python-side validation that ensures the object is associated with only one parent at a time.

  • merge - This cascade indicates that the :meth:`.Session.merge` operation should be propagated from a parent that's the subject of the :meth:`.Session.merge` call down to referred objects. This cascade is also on by default.

  • refresh-expire - A less common option, indicates that the :meth:`.Session.expire` operation should be propagated from a parent down to referred objects. When using :meth:`.Session.refresh`, the referred objects are expired only, but not actually refreshed.

  • expunge - Indicate that when the parent object is removed from the :class:`.Session` using :meth:`.Session.expunge`, the operation should be propagated down to referred objects.

Controlling Cascade on Backrefs

The save-update cascade takes place on backrefs by default. This means that, given a mapping such as this:

mapper(Order, order_table, properties={
    'items' : relationship(Item, backref='order')
})

If an Order is already in the session, and is assigned to the order attribute of an Item, the backref appends the Order to the items collection of that Order, resulting in the save-update cascade taking place:

>>> o1 = Order()
>>> session.add(o1)
>>> o1 in session
True

>>> i1 = Item()
>>> i1.order = o1
>>> i1 in o1.items
True
>>> i1 in session
True

This behavior can be disabled using the cascade_backrefs flag:

mapper(Order, order_table, properties={
    'items' : relationship(Item, backref='order',
                                cascade_backrefs=False)
})

So above, the assignment of i1.order = o1 will append i1 to the items collection of o1, but will not add i1 to the session. You can, of course, :func:`~.Session.add` i1 to the session at a later point. This option may be helpful for situations where an object needs to be kept out of a session until it's construction is completed, but still needs to be given associations to objects which are already persistent in the target session.

Managing Transactions

A newly constructed :class:`.Session` may be said to be in the "begin" state. In this state, the :class:`.Session` has not established any connection or transactional state with any of the :class:`.Engine` objects that may be associated with it.

The :class:`.Session` then receives requests to operate upon a database connection. Typically, this means it is called upon to execute SQL statements using a particular :class:`.Engine`, which may be via :meth:`.Session.query`, :meth:`.Session.execute`, or within a flush operation of pending data, which occurs when such state exists and :meth:`.Session.commit` or :meth:`.Session.flush` is called.

As these requests are received, each new :class:`.Engine` encountered is associated with an ongoing transactional state maintained by the :class:`.Session`. When the first :class:`.Engine` is operated upon, the :class:`.Session` can be said to have left the "begin" state and entered "transactional" state. For each :class:`.Engine` encountered, a :class:`.Connection` is associated with it, which is acquired via the :meth:`.Engine.contextual_connect` method. If a :class:`.Connection` was directly associated with the :class:`.Session` (see :ref:`session_external_transaction` for an example of this), it is added to the transactional state directly.

For each :class:`.Connection`, the :class:`.Session` also maintains a :class:`.Transaction` object, which is acquired by calling :meth:`.Connection.begin` on each :class:`.Connection`, or if the :class:`.Session` object has been established using the flag twophase=True, a :class:`.TwoPhaseTransaction` object acquired via :meth:`.Connection.begin_twophase`. These transactions are all committed or rolled back corresponding to the invocation of the :meth:`.Session.commit` and :meth:`.Session.rollback` methods. A commit operation will also call the :meth:`.TwoPhaseTransaction.prepare` method on all transactions if applicable.

When the transactional state is completed after a rollback or commit, the :class:`.Session` releases all :class:`.Transaction` and :class:`.Connection` resources (which has the effect of returning DBAPI connections to the connection pool of each :class:`.Engine`), and goes back to the "begin" state, which will again invoke new :class:`.Connection` and :class:`.Transaction` objects as new requests to emit SQL statements are received.

The example below illustrates this lifecycle:

engine = create_engine("...")
Session = sessionmaker(bind=engine)

# new session.   no connections are in use.
session = Session()
try:
    # first query.  a Connection is acquired
    # from the Engine, and a Transaction
    # started.
    item1 = session.query(Item).get(1)

    # second query.  the same Connection/Transaction
    # are used.
    item2 = session.query(Item).get(2)

    # pending changes are created.
    item1.foo = 'bar'
    item2.bar = 'foo'

    # commit.  The pending changes above
    # are flushed via flush(), the Transaction
    # is committed, the Connection object closed
    # and discarded, the underlying DBAPI connection
    # returned to the connection pool.
    session.commit()
except:
    # on rollback, the same closure of state
    # as that of commit proceeds.
    session.rollback()
    raise

Using SAVEPOINT

SAVEPOINT transactions, if supported by the underlying engine, may be delineated using the :func:`~sqlalchemy.orm.session.Session.begin_nested` method:

Session = sessionmaker()
session = Session()
session.add(u1)
session.add(u2)

session.begin_nested() # establish a savepoint
session.add(u3)
session.rollback()  # rolls back u3, keeps u1 and u2

session.commit() # commits u1 and u2

:func:`~sqlalchemy.orm.session.Session.begin_nested` may be called any number of times, which will issue a new SAVEPOINT with a unique identifier for each call. For each :func:`~sqlalchemy.orm.session.Session.begin_nested` call, a corresponding :func:`~sqlalchemy.orm.session.Session.rollback` or :func:`~sqlalchemy.orm.session.Session.commit` must be issued.

When :func:`~sqlalchemy.orm.session.Session.begin_nested` is called, a :func:`~sqlalchemy.orm.session.Session.flush` is unconditionally issued (regardless of the autoflush setting). This is so that when a :func:`~sqlalchemy.orm.session.Session.rollback` occurs, the full state of the session is expired, thus causing all subsequent attribute/instance access to reference the full state of the :class:`~sqlalchemy.orm.session.Session` right before :func:`~sqlalchemy.orm.session.Session.begin_nested` was called.

:meth:`~.Session.begin_nested`, in the same manner as the less often used :meth:`~.Session.begin` method, returns a transactional object which also works as a context manager. It can be succinctly used around individual record inserts in order to catch things like unique constraint exceptions:

for record in records:
    try:
        with session.begin_nested():
            session.merge(record)
    except:
        print "Skipped record %s" % record
session.commit()

Autocommit Mode

The example of :class:`.Session` transaction lifecycle illustrated at the start of :ref:`unitofwork_transaction` applies to a :class:`.Session` configured in the default mode of autocommit=False. Constructing a :class:`.Session` with autocommit=True produces a :class:`.Session` placed into "autocommit" mode, where each SQL statement invoked by a :meth:`.Session.query` or :meth:`.Session.execute` occurs using a new connection from the connection pool, discarding it after results have been iterated. The :meth:`.Session.flush` operation still occurs within the scope of a single transaction, though this transaction is closed out after the :meth:`.Session.flush` operation completes.

"autocommit" mode should not be considered for general use. While very old versions of SQLAlchemy standardized on this mode, the modern :class:`.Session` benefits highly from being given a clear point of transaction demarcation via :meth:`.Session.rollback` and :meth:`.Session.commit`. The autoflush action can safely emit SQL to the database as needed without implicitly producing permanent effects, the contents of attributes are expired only when a logical series of steps has completed. If the :class:`.Session` were to be used in pure "autocommit" mode without an ongoing transaction, these features should be disabled, that is, autoflush=False, expire_on_commit=False.

Modern usage of "autocommit" is for framework integrations that need to control specifically when the "begin" state occurs. A session which is configured with autocommit=True may be placed into the "begin" state using the :meth:`.Session.begin` method. After the cycle completes upon :meth:`.Session.commit` or :meth:`.Session.rollback`, connection and transaction resources are released and the :class:`.Session` goes back into "autocommit" mode, until :meth:`.Session.begin` is called again:

Session = sessionmaker(bind=engine, autocommit=True)
session = Session()
session.begin()
try:
    item1 = session.query(Item).get(1)
    item2 = session.query(Item).get(2)
    item1.foo = 'bar'
    item2.bar = 'foo'
    session.commit()
except:
    session.rollback()
    raise

The :func:`.Session.begin` method also returns a transactional token which is compatible with the Python 2.6 with statement:

Session = sessionmaker(bind=engine, autocommit=True)
session = Session()
with session.begin():
    item1 = session.query(Item).get(1)
    item2 = session.query(Item).get(2)
    item1.foo = 'bar'
    item2.bar = 'foo'

Using Subtransactions with Autocommit

A subtransaction indicates usage of the :meth:`.Session.begin` method in conjunction with the subtransactions=True flag. This produces a a non-transactional, delimiting construct that allows nesting of calls to :meth:`~.Session.begin` and :meth:`~.Session.commit`. It's purpose is to allow the construction of code that can function within a transaction both independently of any external code that starts a transaction, as well as within a block that has already demarcated a transaction.

subtransactions=True is generally only useful in conjunction with autocommit, and is equivalent to the pattern described at :ref:`connections_nested_transactions`, where any number of functions can call :meth:`.Connection.begin` and :meth:`.Transaction.commit` as though they are the initiator of the transaction, but in fact may be participating in an already ongoing transaction:

# method_a starts a transaction and calls method_b
def method_a(session):
    session.begin(subtransactions=True)
    try:
        method_b(session)
        session.commit()  # transaction is committed here
    except:
        session.rollback() # rolls back the transaction
        raise

# method_b also starts a transaction, but when
# called from method_a participates in the ongoing
# transaction.
def method_b(session):
    session.begin(subtransactions=True)
    try:
        session.add(SomeObject('bat', 'lala'))
        session.commit()  # transaction is not committed yet
    except:
        session.rollback() # rolls back the transaction, in this case
                           # the one that was initiated in method_a().
        raise

# create a Session and call method_a
session = Session(autocommit=True)
method_a(session)
session.close()

Subtransactions are used by the :meth:`.Session.flush` process to ensure that the flush operation takes place within a transaction, regardless of autocommit. When autocommit is disabled, it is still useful in that it forces the :class:`.Session` into a "pending rollback" state, as a failed flush cannot be resumed in mid-operation, where the end user still maintains the "scope" of the transaction overall.

Enabling Two-Phase Commit

For backends which support two-phase operaration (currently MySQL and PostgreSQL), the session can be instructed to use two-phase commit semantics. This will coordinate the committing of transactions across databases so that the transaction is either committed or rolled back in all databases. You can also :func:`~sqlalchemy.orm.session.Session.prepare` the session for interacting with transactions not managed by SQLAlchemy. To use two phase transactions set the flag twophase=True on the session:

engine1 = create_engine('postgresql://db1')
engine2 = create_engine('postgresql://db2')

Session = sessionmaker(twophase=True)

# bind User operations to engine 1, Account operations to engine 2
Session.configure(binds={User:engine1, Account:engine2})

session = Session()

# .... work with accounts and users

# commit.  session will issue a flush to all DBs, and a prepare step to all DBs,
# before committing both transactions
session.commit()

Embedding SQL Insert/Update Expressions into a Flush

This feature allows the value of a database column to be set to a SQL expression instead of a literal value. It's especially useful for atomic updates, calling stored procedures, etc. All you do is assign an expression to an attribute:

class SomeClass(object):
    pass
mapper(SomeClass, some_table)

someobject = session.query(SomeClass).get(5)

# set 'value' attribute to a SQL expression adding one
someobject.value = some_table.c.value + 1

# issues "UPDATE some_table SET value=value+1"
session.commit()

This technique works both for INSERT and UPDATE statements. After the flush/commit operation, the value attribute on someobject above is expired, so that when next accessed the newly generated value will be loaded from the database.

Using SQL Expressions with Sessions

SQL expressions and strings can be executed via the :class:`~sqlalchemy.orm.session.Session` within its transactional context. This is most easily accomplished using the :func:`~sqlalchemy.orm.session.Session.execute` method, which returns a :class:`~sqlalchemy.engine.ResultProxy` in the same manner as an :class:`~sqlalchemy.engine.Engine` or :class:`~sqlalchemy.engine.Connection`:

Session = sessionmaker(bind=engine)
session = Session()

# execute a string statement
result = session.execute("select * from table where id=:id", {'id':7})

# execute a SQL expression construct
result = session.execute(select([mytable]).where(mytable.c.id==7))

The current :class:`~sqlalchemy.engine.Connection` held by the :class:`~sqlalchemy.orm.session.Session` is accessible using the :func:`~sqlalchemy.orm.session.Session.connection` method:

connection = session.connection()

The examples above deal with a :class:`~sqlalchemy.orm.session.Session` that's bound to a single :class:`~sqlalchemy.engine.Engine` or :class:`~sqlalchemy.engine.Connection`. To execute statements using a :class:`~sqlalchemy.orm.session.Session` which is bound either to multiple engines, or none at all (i.e. relies upon bound metadata), both :func:`~sqlalchemy.orm.session.Session.execute` and :func:`~sqlalchemy.orm.session.Session.connection` accept a mapper keyword argument, which is passed a mapped class or :class:`~sqlalchemy.orm.mapper.Mapper` instance, which is used to locate the proper context for the desired engine:

Session = sessionmaker()
session = Session()

# need to specify mapper or class when executing
result = session.execute("select * from table where id=:id", {'id':7}, mapper=MyMappedClass)

result = session.execute(select([mytable], mytable.c.id==7), mapper=MyMappedClass)

connection = session.connection(MyMappedClass)

Joining a Session into an External Transaction

If a :class:`.Connection` is being used which is already in a transactional state (i.e. has a :class:`.Transaction` established), a :class:`.Session` can be made to participate within that transaction by just binding the :class:`.Session` to that :class:`.Connection`. The usual rationale for this is a test suite that allows ORM code to work freely with a :class:`.Session`, including the ability to call :meth:`.Session.commit`, where afterwards the entire database interaction is rolled back:

from sqlalchemy.orm import sessionmaker
from sqlalchemy import create_engine
from unittest import TestCase

# global application scope.  create Session class, engine
Session = sessionmaker()

engine = create_engine('postgresql://...')

class SomeTest(TestCase):
    def setUp(self):
        # connect to the database
        self.connection = engine.connect()

        # begin a non-ORM transaction
        self.trans = connection.begin()

        # bind an individual Session to the connection
        self.session = Session(bind=self.connection)

    def test_something(self):
        # use the session in tests.

        self.session.add(Foo())
        self.session.commit()

    def tearDown(self):
        # rollback - everything that happened with the
        # Session above (including calls to commit())
        # is rolled back.
        self.trans.rollback()
        self.session.close()

Above, we issue :meth:`.Session.commit` as well as :meth:`.Transaction.rollback`. This is an example of where we take advantage of the :class:`.Connection` object's ability to maintain subtransactions, or nested begin/commit-or-rollback pairs where only the outermost begin/commit pair actually commits the transaction, or if the outermost block rolls back, everything is rolled back.

Contextual/Thread-local Sessions

Recall from the section :ref:`session_faq_whentocreate`, the concept of "session scopes" was introduced, with an emphasis on web applications and the practice of linking the scope of a :class:`.Session` with that of a web request. Most modern web frameworks include integration tools so that the scope of the :class:`.Session` can be managed automatically, and these tools should be used as they are available.

SQLAlchemy includes its own helper object, which helps with the establishment of user-defined :class:`.Session` scopes. It is also used by third-party integration systems to help construct their integration schemes.

The object is the :class:`.scoped_session` object, and it represents a registry of :class:`.Session` objects. If you're not familiar with the registry pattern, a good introduction can be found in Patterns of Enterprise Architecture.

Note

The :class:`.scoped_session` object is a very popular and useful object used by many SQLAlchemy applications. However, it is important to note that it presents only one approach to the issue of :class:`.Session` management. If you're new to SQLAlchemy, and especially if the term "thread-local variable" seems strange to you, we recommend that if possible you familiarize first with an off-the-shelf integration system such as Flask-SQLAlchemy or zope.sqlalchemy.

A :class:`.scoped_session` is constructed by calling it, passing it a factory which can create new :class:`.Session` objects. A factory is just something that produces a new object when called, and in the case of :class:`.Session`, the most common factory is the :class:`.sessionmaker`, introduced earlier in this section. Below we illustrate this usage:

>>> from sqlalchemy.orm import scoped_session
>>> from sqlalchemy.orm import sessionmaker

>>> session_factory = sessionmaker(bind=some_engine)
>>> Session = scoped_session(session_factory)

The :class:`.scoped_session` object we've created will now call upon the :class:`.sessionmaker` when we "call" the registry:

>>> some_session = Session()

Above, some_session is an instance of :class:`.Session`, which we can now use to talk to the database. This same :class:`.Session` is also present within the :class:`.scoped_session` registry we've created. If we call upon the registry a second time, we get back the same :class:`.Session`:

>>> some_other_session = Session()
>>> some_session is some_other_session
True

This pattern allows disparate sections of the application to call upon a global :class:`.scoped_session`, so that all those areas may share the same session without the need to pass it explicitly. The :class:`.Session` we've established in our registry will remain, until we explicitly tell our regsitry to dispose of it, by calling :meth:`.scoped_session.remove`:

>>> Session.remove()

The :meth:`.scoped_session.remove` method first calls :meth:`.Session.close` on the current :class:`.Session`, which has the effect of releasing any connection/transactional resources owned by the :class:`.Session` first, then discarding the :class:`.Session` itself. "Releasing" here means that any pending transaction will be rolled back using connection.rollback().

At this point, the :class:`.scoped_session` object is "empty", and will create a new :class:`.Session` when called again. As illustrated below, this is not the same :class:`.Session` we had before:

>>> new_session = Session()
>>> new_session is some_session
False

The above series of steps illustrates the idea of the "registry" pattern in a nutshell. With that basic idea in hand, we can discuss some of the details of how this pattern proceeds.

Implicit Method Access

The job of the :class:`.scoped_session` is simple; hold onto a :class:`.Session` for all who ask for it. As a means of producing more transparent access to this :class:`.Session`, the :class:`.scoped_session` also includes proxy behavior, meaning that the registry itself can be treated just like a :class:`.Session` directly; when methods are called on this object, they are proxied to the underlying :class:`.Session` being maintained by the registry:

Session = scoped_session(some_factory)

# equivalent to:
#
# session = Session()
# print session.query(MyClass).all()
#
print Session.query(MyClass).all()

The above code accomplishes the same task as that of acquiring the current :class:`.Session` by calling upon the registry, then using that :class:`.Session`.

Thread-Local Scope

Users who are familiar with multithreaded programming will note that representing anything as a global variable is usually a bad idea, as it implies that the global object will be accessed by many threads concurrently. The :class:`.Session` object is entirely designed to be used in a non-concurrent fashion, which in terms of multithreading means "only in one thread at a time". So our above example of :class:`.scoped_session` usage, where the same :class:`.Session` object is maintained across multiple calls, suggests that some process needs to be in place such that mutltiple calls across many threads don't actually get a handle to the same session. We call this notion thread local storage, which means, a special object is used that will maintain a distinct object per each application thread. Python provides this via the threading.local() construct. The :class:`.scoped_session` object by default uses this object as storage, so that a single :class:`.Session` is maintained for all who call upon the :class:`.scoped_session` registry, but only within the scope of a single thread. Callers who call upon the registry in a different thread get a :class:`.Session` instance that is local to that other thread.

Using this technique, the :class:`.scoped_session` provides a quick and relatively simple (if one is familiar with thread-local storage) way of providing a single, global object in an application that is safe to be called upon from multiple threads.

The :meth:`.scoped_session.remove` method, as always, removes the current :class:`.Session` associated with the thread, if any. However, one advantage of the threading.local() object is that if the application thread itself ends, the "storage" for that thread is also garbage collected. So it is in fact "safe" to use thread local scope with an application that spawns and tears down threads, without the need to call :meth:`.scoped_session.remove`. However, the scope of transactions themselves, i.e. ending them via :meth:`.Session.commit` or :meth:`.Session.rollback`, will usually still be something that must be explicitly arranged for at the appropriate time, unless the application actually ties the lifespan of a thread to the lifespan of a transaction.

Using Thread-Local Scope with Web Applications

As discussed in the section :ref:`session_faq_whentocreate`, a web application is architected around the concept of a web request, and integrating such an application with the :class:`.Session` usually implies that the :class:`.Session` will be associated with that request. As it turns out, most Python web frameworks, with notable exceptions such as the asynchronous frameworks Twisted and Tornado, use threads in a simple way, such that a particular web request is received, processed, and completed within the scope of a single worker thread. When the request ends, the worker thread is released to a pool of workers where it is available to handle another request.

This simple correspondence of web request and thread means that to associate a :class:`.Session` with a thread implies it is also associated with the web request running within that thread, and vice versa, provided that the :class:`.Session` is created only after the web request begins and torn down just before the web request ends. So it is a common practice to use :class:`.scoped_session` as a quick way to integrate the :class:`.Session` with a web application. The sequence diagram below illustrates this flow:

Web Server          Web Framework        SQLAlchemy ORM Code
--------------      --------------       ------------------------------
startup        ->   Web framework        # Session registry is established
                    initializes          Session = scoped_session(sessionmaker())

incoming
web request    ->   web request     ->   # The registry is *optionally*
                    starts               # called upon explicitly to create
                                         # a Session local to the thread and/or request
                                         Session()

                                         # the Session registry can otherwise
                                         # be used at any time, creating the
                                         # request-local Session() if not present,
                                         # or returning the existing one
                                         Session.query(MyClass) # ...

                                         Session.add(some_object) # ...

                                         # if data was modified, commit the
                                         # transaction
                                         Session.commit()

                    web request ends  -> # the registry is instructed to
                                         # remove the Session
                                         Session.remove()

                    sends output      <-
outgoing web    <-
response

Using the above flow, the process of integrating the :class:`.Session` with the web application has exactly two requirements:

  1. Create a single :class:`.scoped_session` registry when the web application first starts, ensuring that this object is accessible by the rest of the application.
  2. Ensure that :meth:`.scoped_session.remove` is called when the web request ends, usually by integrating with the web framework's event system to establish an "on request end" event.

As noted earlier, the above pattern is just one potential way to integrate a :class:`.Session` with a web framework, one which in particular makes the significant assumption that the web framework associates web requests with application threads. It is however strongly recommended that the integration tools provided with the web framework itself be used, if available, instead of :class:`.scoped_session`.

In particular, while using a thread local can be convenient, it is preferable that the :class:`.Session` be associated directly with the request, rather than with the current thread. The next section on custom scopes details a more advanced configuration which can combine the usage of :class:`.scoped_session` with direct request based scope, or any kind of scope.

Using Custom Created Scopes

The :class:`.scoped_session` object's default behavior of "thread local" scope is only one of many options on how to "scope" a :class:`.Session`. A custom scope can be defined based on any existing system of getting at "the current thing we are working with".

Suppose a web framework defines a library function get_current_request(). An application built using this framework can call this function at any time, and the result will be some kind of Request object that represents the current request being processed. If the Request object is hashable, then this function can be easily integrated with :class:`.scoped_session` to associate the :class:`.Session` with the request. Below we illustrate this in conjunction with a hypothetical event marker provided by the web framework on_request_end, which allows code to be invoked whenever a request ends:

from my_web_framework import get_current_request, on_request_end
from sqlalchemy.orm import scoped_session, sessionmaker

Session = scoped_session(sessionmaker(bind=some_engine), scopefunc=get_current_request)

@on_request_end
def remove_session(req):
    Session.remove()

Above, we instantiate :class:`.scoped_session` in the usual way, except that we pass our request-returning function as the "scopefunc". This instructs :class:`.scoped_session` to use this function to generate a dictionary key whenever the registry is called upon to return the current :class:`.Session`. In this case it is particularly important that we ensure a reliable "remove" system is implemented, as this dictionary is not otherwise self-managed.

Contextual Session API

Partitioning Strategies

Simple Vertical Partitioning

Vertical partitioning places different kinds of objects, or different tables, across multiple databases:

engine1 = create_engine('postgresql://db1')
engine2 = create_engine('postgresql://db2')

Session = sessionmaker(twophase=True)

# bind User operations to engine 1, Account operations to engine 2
Session.configure(binds={User:engine1, Account:engine2})

session = Session()

Above, operations against either class will make usage of the :class:`.Engine` linked to that class. Upon a flush operation, similar rules take place to ensure each class is written to the right database.

The transactions among the multiple databases can optionally be coordinated via two phase commit, if the underlying backend supports it. See :ref:`session_twophase` for an example.

Custom Vertical Partitioning

More comprehensive rule-based class-level partitioning can be built by overriding the :meth:`.Session.get_bind` method. Below we illustrate a custom :class:`.Session` which delivers the following rules:

  1. Flush operations are delivered to the engine named master.
  2. Operations on objects that subclass MyOtherClass all occur on the other engine.
  3. Read operations for all other classes occur on a random choice of the slave1 or slave2 database.
engines = {
    'master':create_engine("sqlite:///master.db"),
    'other':create_engine("sqlite:///other.db"),
    'slave1':create_engine("sqlite:///slave1.db"),
    'slave2':create_engine("sqlite:///slave2.db"),
}

from sqlalchemy.orm import Session, sessionmaker
import random

class RoutingSession(Session):
    def get_bind(self, mapper=None, clause=None):
        if mapper and issubclass(mapper.class_, MyOtherClass):
            return engines['other']
        elif self._flushing:
            return engines['master']
        else:
            return engines[
                random.choice(['slave1','slave2'])
            ]

The above :class:`.Session` class is plugged in using the class_ argument to :class:`.sessionmaker`:

Session = sessionmaker(class_=RoutingSession)

This approach can be combined with multiple :class:`.MetaData` objects, using an approach such as that of using the declarative __abstract__ keyword, described at :ref:`declarative_abstract`.

Horizontal Partitioning

Horizontal partitioning partitions the rows of a single table (or a set of tables) across multiple databases.

See the "sharding" example: :ref:`examples_sharding`.

Sessions API

Session and sessionmaker()

Session Utilites

Attribute and State Management Utilities

These functions are provided by the SQLAlchemy attribute instrumentation API to provide a detailed interface for dealing with instances, attribute values, and history. Some of them are useful when constructing event listener functions, such as those described in :ref:`events_orm_toplevel`.