.. _session_toplevel:

=================
Using the Session
=================

.. module:: sqlalchemy.orm.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
<http://martinfowler.com/eaaCatalog/identityMap.html>`_ - 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 <http://martinfowler.com/eaaCatalog/unitOfWork.html>`_ 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 :func:`.sessionmaker` function 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 :func:`.sessionmaker` is illustrated below:

.. sourcecode:: python+sql

    from sqlalchemy.orm import sessionmaker

    # 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 :func:`.sessionmaker` call creates a class for us,
which we assign to the name ``Session``. This class is a subclass of the
actual :class:`.Session` class, which when instantiated, will
use the arguments we've given the function, in this case
to use a particular :class:`.Engine` for connection resources.

When you write your application, place the call to
:func:`.sessionmaker` somewhere global, and then make your new
``Session`` class available to the rest of your application.

A typical setup will associate the :func:`.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.   You 
can also associate a :class:`.Engine` with an existing :func:`.sessionmaker` 
using the :meth:`.sessionmaker.configure` method::

    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()

you can also associate individual :class:`.Session` objects with an :class:`.Engine`
on each invocation::

    session = Session(bind=engine)

...or directly with a :class:`.Connection`::

    conn = engine.connect()
    session = Session(bind=conn)

While the rationale for the above example may not be apparent, the typical
usage is in a test fixture that maintains an external transaction - see
:ref:`session_external_transaction` below for a full example.

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:`~sqlalchemy.orm.session.Session` tries to be strict about ambiguous
operations (such as trying to save the same object to two different sessions
at the same time).

Frequently Asked Questions
--------------------------

* When do I make a :func:`.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 :func:`.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 ``configure()``.

    In the examples in this section, we will frequently show the
    :func:`.sessionmaker` being created right above the line where we actually
    invoke :class:`~sqlalchemy.orm.session.Session()`. But that's just for
    example's sake ! In reality, the :func:`.sessionmaker` would be somewhere
    at the module level, and your individual
    :class:`~sqlalchemy.orm.session.Session()` calls would be sprinkled all
    throughout your app, such as in a web application within each controller
    method.

* When do I make a :class:`.Session` ?

    You typically invoke :class:`.Session` when you first need to talk to your
    database, and want to save some objects or load some existing ones. It
    then remains in use for the lifespan of a particular database
    conversation, which includes not just the initial loading of objects but
    throughout the whole usage of those instances.

    Objects become detached if their owning session is discarded. They are
    still functional in the detached state if the user has ensured that their
    state has not been expired before detachment, but they will not be able to
    represent the current state of database data. Because of this, it's best
    to consider persisted objects as an extension of the state of a particular
    :class:`.Session`, and to keep that session around until all referenced
    objects have been discarded.

    An exception to this is when objects are placed in caches or otherwise
    shared among threads or processes, in which case their detached state can
    be stored, transmitted, or shared. However, the state of detached objects
    should still be transferred back into a new :class:`.Session` using
    :meth:`.Session.add` or :meth:`.Session.merge` before working with the
    object (or in the case of merge, its state) again.

    It is also very common that a :class:`.Session` as well as its associated
    objects are only referenced by a single thread.  Sharing objects between
    threads is most safely accomplished by sharing their state among multiple
    instances of those objects, each associated with a distinct
    :class:`.Session` per thread, :meth:`.Session.merge` to transfer state
    between threads.   This pattern is not a strict requirement by any means, 
    but it has the least chance of introducing concurrency issues.

    To help with the recommended :class:`.Session` -per-thread,
    :class:`.Session` -per-set-of-objects patterns, the
    :func:`.scoped_session` function is provided which produces a
    thread-managed registry of :class:`.Session` objects. It is commonly used
    in web applications so that a single global variable can be used to safely
    represent transactional sessions with sets of objects, localized to a
    single thread. More on this object is in :ref:`unitofwork_contextual`.

* 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 <http://beaker.groovie.org/>`_, 
    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)

.. index::
   single: thread safety; sessions
   single: thread safety; Session

* Is the session thread-safe?

    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
    :func:`.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`.

.. _unitofwork_merging:

Merging
-------

:func:`~sqlalchemy.orm.session.Session.merge` reconciles the current state of
an instance and its associated children with existing data in the database,
and returns a copy of the instance associated with the session. 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 load an instance with that primary key (or pulls from the local
  identity map).
* If there's no primary key on the given instance, or the given primary key
  does not exist in the database, a new instance is created.
* The state of the given instance is then copied onto the located/newly
  created instance.
* The operation is cascaded to associated child items along the ``merge``
  cascade. Note that all changes present on the given instance, including
  changes to collections, are merged.
* The new instance is returned.

With :func:`~sqlalchemy.orm.session.Session.merge`, the given instance is not
placed within the session, and can be associated with a different session or
detached. :func:`~sqlalchemy.orm.session.Session.merge` is very useful for
taking the state of any kind of object structure without regard for its
origins or current session associations and placing that state within a
session. Here's two 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
  :func:`~sqlalchemy.orm.session.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.
* A web application stores mapped entities within an HTTP session object.
  When each request starts up, the serialized data can be
  merged into the session, so that the original entity may
  be safely shared among requests and threads.

:func:`~sqlalchemy.orm.session.Session.merge` is frequently used by
applications which implement their own second level caches. This refers to an
application which uses an in memory dictionary, or an tool like Memcached to
store objects over long running spans of time. When such an object needs to
exist within a :class:`~sqlalchemy.orm.session.Session`,
:func:`~sqlalchemy.orm.session.Session.merge` is a good choice since it leaves
the original cached object untouched. For this use case, merge provides a
keyword option called ``load=False``. When this boolean flag is set to
``False``, :func:`~sqlalchemy.orm.session.Session.merge` will not issue any
SQL to reconcile the given object against the current state of the database,
thereby reducing query overhead. The limitation is that the given object and
all of its children may not contain any pending changes, and it's also of
course possible that newer information in the database will not be present on
the merged object, since no load is issued.

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.

Note that a new :func:`.relationship` option introduced in 0.6.5, 
``cascade_backrefs=False``, 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 ? 

.. sourcecode:: python+sql

    >>> 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 :func:`~sqlalchemy.orm.session.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()

The big gotcha with :func:`~sqlalchemy.orm.session.Session.delete` is that
**nothing is removed from collections**. Such as, if a ``User`` has a
collection of three ``Addresses``, deleting an ``Address`` will not remove it
from ``user.addresses``::

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

The solution is to use proper cascading::

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

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
:func:`.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
<http://www.sqlalchemy.org/trac/wiki/UsageRecipes/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.

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
:func:`.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:

.. sourcecode:: python+sql

    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

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
:func:`.sessionmaker` with the ``weak_identity_map=False``
setting.

.. _unitofwork_cascades:

Cascades
========

Mappers support the concept of configurable *cascade* behavior on
:func:`~sqlalchemy.orm.relationship` constructs. This behavior controls how
the Session should treat the instances that have a parent-child relationship
with another instance that is operated upon by the Session. Cascade is
indicated as a comma-separated list of string keywords, with the possible
values ``all``, ``delete``, ``save-update``, ``refresh-expire``, ``merge``,
``expunge``, and ``delete-orphan``.

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

    mapper(Order, order_table, properties={
        'items' : relationship(Item, items_table, cascade="all, delete-orphan"),
        'customer' : relationship(User, users_table, user_orders_table, cascade="save-update"),
    })

The above mapper specifies two relationships, ``items`` and ``customer``. The
``items`` relationship specifies "all, delete-orphan" as its ``cascade``
value, indicating that all ``add``, ``merge``, ``expunge``, ``refresh``
``delete`` and ``expire`` operations performed on a parent ``Order`` instance
should also be performed on the child ``Item`` instances attached to it. The
``delete-orphan`` cascade value additionally indicates that if an ``Item``
instance is no longer associated with an ``Order``, it should also be deleted.
The "all, delete-orphan" cascade argument allows a so-called *lifecycle*
relationship between an ``Order`` and an ``Item`` object.

The ``customer`` relationship specifies only the "save-update" cascade value,
indicating most operations will not be cascaded from a parent ``Order``
instance to a child ``User`` instance except for the
:func:`~sqlalchemy.orm.session.Session.add` operation. ``save-update`` cascade
indicates that an :func:`~sqlalchemy.orm.session.Session.add` on the parent
will cascade to all child items, and also that items added to a parent which
is already present in a session will also be added to that same session.
"save-update" cascade also cascades the *pending history* of a
relationship()-based 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 new session - this so that foreign key clear operations
and deletions will take place (new in 0.6).

Note that the ``delete-orphan`` cascade only functions for relationships where
the target object can have a single parent at a time, meaning it is only
appropriate for one-to-one or one-to-many relationships. For a
:func:`~sqlalchemy.orm.relationship` which establishes one-to-one via a local
foreign key, i.e. a many-to-one that stores only a single parent, or
one-to-one/one-to-many via a "secondary" (association) table, a warning will
be issued if ``delete-orphan`` is configured. To disable this warning, also
specify the ``single_parent=True`` flag on the relationship, which constrains
objects to allow attachment to only one parent at a time.

The default value for ``cascade`` on :func:`~sqlalchemy.orm.relationship` is
``save-update, merge``.

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

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

If an ``Order`` is already in the session, and is assigned to the ``order``
attribute of an ``Item``, the backref appends the ``Item`` to the ``orders``
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.orders
    True
    >>> i1 in session
    True

This behavior can be disabled as of 0.6.5 using the ``cascade_backrefs`` flag::

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

So above, the assignment of ``i1.order = o1`` will append ``i1`` to the ``orders``
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.


.. _unitofwork_transaction:

Managing Transactions
=====================

The :class:`~sqlalchemy.orm.session.Session` manages transactions across all
engines associated with it. As the :class:`~sqlalchemy.orm.session.Session`
receives requests to execute SQL statements using a particular
:class:`~sqlalchemy.engine.base.Engine` or
:class:`~sqlalchemy.engine.base.Connection`, it adds each individual
:class:`~sqlalchemy.engine.base.Engine` encountered to its transactional state
and maintains an open connection for each one (note that a simple application
normally has just one :class:`~sqlalchemy.engine.base.Engine`). At commit
time, all unflushed data is flushed, and each individual transaction is
committed. If the underlying databases support two-phase semantics, this may
be used by the Session as well if two-phase transactions are enabled.

Normal operation ends the transactional state using the
:func:`~sqlalchemy.orm.session.Session.rollback` or
:func:`~sqlalchemy.orm.session.Session.commit` methods. After either is
called, the :class:`~sqlalchemy.orm.session.Session` starts a new
transaction::

    Session = sessionmaker()
    session = Session()
    try:
        item1 = session.query(Item).get(1)
        item2 = session.query(Item).get(2)
        item1.foo = 'bar'
        item2.bar = 'foo'

        # commit- will immediately go into 
        # a new transaction on next use.
        session.commit()
    except:
        # rollback - will immediately go into 
        # a new transaction on next use.
        session.rollback()

A session which is configured with ``autocommit=True`` may be placed into a
transaction using :func:`~sqlalchemy.orm.session.Session.begin`. With an
``autocommit=True`` session that's been placed into a transaction using
:func:`~sqlalchemy.orm.session.Session.begin`, the session releases all
connection resources after a :func:`~sqlalchemy.orm.session.Session.commit` or
:func:`~sqlalchemy.orm.session.Session.rollback` and remains transaction-less
(with the exception of flushes) until the next
:func:`~sqlalchemy.orm.session.Session.begin` call::

    Session = sessionmaker(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:`~sqlalchemy.orm.session.Session.begin` method also returns a
transactional token which is compatible with the Python 2.6 ``with``
statement::

    Session = sessionmaker(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'

.. _session_begin_nested:

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.

.. _session_subtransactions:

Using Subtransactions
---------------------

A subtransaction, as offered by the ``subtransactions=True`` flag of :meth:`.Session.begin`,
is 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.  By "non-transactional", we
mean that no actual transactional dialogue with the database is generated by this flag beyond that of
a single call to :meth:`~.Session.begin`, regardless of how many times the method
is called within a transaction.

The subtransaction feature is in fact intrinsic to any call to :meth:`~.Session.flush`, which uses
it internally to ensure that the series of flush steps are enclosed within a transaction,
regardless of the setting of ``autocommit`` or the presence of an existing transactional context.
However, explicit usage of the ``subtransactions=True`` flag is generally only useful with an 
application that uses the
:class:`.Session` in "autocommit=True" mode, and calls :meth:`~.Session.begin` explicitly
in order to demarcate transactions.  For this reason the subtransaction feature is not
commonly used in an explicit way, except for apps that integrate SQLAlchemy-level transaction control with
the transaction control of another library or subsystem.  For true, general purpose "nested" 
transactions, where a rollback affects only a portion of the work which has proceeded, 
savepoints should be used, documented in :ref:`session_begin_nested`.

The feature is the ORM 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.

As is the case with the non-ORM :class:`.Transaction` object,
calling :meth:`.Session.rollback` rolls back the **entire**
transaction, which was initiated by the first call to
:meth:`.Session.begin` (whether this call was explicit by the
end user, or implicit in an ``autocommit=False`` scenario).
However, the :class:`.Session` still considers itself to be in a
"partially rolled back" state until :meth:`.Session.rollback` is
called explicitly for each call that was made to
:meth:`.Session.begin`, where "partially rolled back" means that
no further SQL operations can proceed until each level
of the transaction has been acounted for, unless the :meth:`~.Session.close` method
is called which cancels all transactional markers. For a full exposition on 
the rationale for this,
please see "`But why isn't the one automatic call to ROLLBACK
enough ? Why must I ROLLBACK again?
<http://www.sqlalchemy.org/trac/wiki/FAQ#ButwhyisnttheoneautomaticcalltoROLLBACKenoughWhymustIROLLBACKagain>`_".
The general theme is that if subtransactions are used as intended, that is, as a means to nest multiple
begin/commit pairs, the appropriate rollback calls naturally occur in any case, and allow the session's 
nesting of transactional pairs to function in a simple and predictable way 
without the need to guess as to what level is active.

An example of ``subtransactions=True`` is nearly identical to
that of the non-ORM technique. The nesting of transactions, as
well as the natural presence of "rollback" for all transactions
should an exception occur, is illustrated::

    # method_a starts a transaction and calls method_b
    def method_a(session):
        session.begin(subtransactions=True) # open a transaction.  If there was
                                            # no previous call to begin(), this will
                                            # begin a real transaction (meaning, a 
                                            # DBAPI connection is procured, which as
                                            # per the DBAPI specification is in a transactional
                                            # state ready to be committed or rolled back)
        try:
            method_b(session)
            session.commit()  # transaction is committed here
        except:
            session.rollback() # rolls back the transaction
            raise

    # method_b also starts a transaction
    def method_b(connection):
        session.begin(subtransactions=True) # open a transaction - this 
                                            # runs in the context of method_a()'s 
                                            # transaction
        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()

Since the :meth:`.Session.flush` method uses a subtransaction, a failed flush
will always issue a rollback which then affects the state of the outermost transaction (unless a SAVEPOINT
is in use).   This forces the need to issue :meth:`~.Session.rollback` for the full operation
before subsequent SQL operations can proceed.

Enabling Two-Phase Commit
-------------------------

Finally, for MySQL, PostgreSQL, and soon Oracle as well, 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.base.ResultProxy` in the same manner as an
:class:`~sqlalchemy.engine.base.Engine` or
:class:`~sqlalchemy.engine.base.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.base.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.base.Engine` or
:class:`~sqlalchemy.engine.base.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)

.. _session_external_transaction:

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.

The :class:`.Session` object and :func:`.sessionmaker` function
================================================================

.. autofunction:: sessionmaker

.. autoclass:: sqlalchemy.orm.session.Session
   :members:

.. _unitofwork_contextual:

Contextual/Thread-local Sessions
=================================

A common need in applications, particularly those built around web frameworks,
is the ability to "share" a :class:`~sqlalchemy.orm.session.Session` object
among disparate parts of an application, without needing to pass the object
explicitly to all method and function calls. What you're really looking for is
some kind of "global" session object, or at least "global" to all the parts of
an application which are tasked with servicing the current request. For this
pattern, SQLAlchemy provides the ability to enhance the
:class:`~sqlalchemy.orm.session.Session` class generated by
:func:`.sessionmaker` to provide auto-contextualizing support.
This means that whenever you create a :class:`~sqlalchemy.orm.session.Session`
instance with its constructor, you get an *existing*
:class:`~sqlalchemy.orm.session.Session` object which is bound to some
"context". By default, this context is the current thread. This feature is
what previously was accomplished using the ``sessioncontext`` SQLAlchemy
extension.

Creating a Thread-local Context
-------------------------------

The :func:`~sqlalchemy.orm.scoped_session` function wraps around the
:func:`.sessionmaker` function, and produces an object which
behaves the same as the :class:`~sqlalchemy.orm.session.Session` subclass
returned by :func:`.sessionmaker`::

    from sqlalchemy.orm import scoped_session, sessionmaker
    Session = scoped_session(sessionmaker())

However, when you instantiate this :class:`~sqlalchemy.orm.session.Session`
"class", in reality the object is pulled from a threadlocal variable, or if it
doesn't exist yet, it's created using the underlying class generated by
:func:`.sessionmaker`::

    >>> # call Session() the first time.  the new Session instance is created.
    >>> session = Session()

    >>> # later, in the same application thread, someone else calls Session()
    >>> session2 = Session()

    >>> # the two Session objects are *the same* object
    >>> session is session2
    True

Since the :class:`~sqlalchemy.orm.session.Session()` constructor now returns
the same :class:`~sqlalchemy.orm.session.Session` object every time within the
current thread, the object returned by :func:`~sqlalchemy.orm.scoped_session`
also implements most of the :class:`~sqlalchemy.orm.session.Session` methods
and properties at the "class" level, such that you don't even need to
instantiate :class:`~sqlalchemy.orm.session.Session()`::

    # create some objects
    u1 = User()
    u2 = User()

    # save to the contextual session, without instantiating
    Session.add(u1)
    Session.add(u2)

    # view the "new" attribute
    assert u1 in Session.new

    # commit changes
    Session.commit()

The contextual session may be disposed of by calling ``Session.remove()``::

    # remove current contextual session
    Session.remove()

After ``remove()`` is called, the next operation with the contextual session
will start a new :class:`~sqlalchemy.orm.session.Session` for the current
thread.

.. _session_lifespan:

Lifespan of a Contextual Session
--------------------------------

A (really, really) common question is when does the contextual session get
created, when does it get disposed ? We'll consider a typical lifespan as used
in a web application::

    Web Server          Web Framework        User-defined Controller Call
    --------------      --------------       ------------------------------
    web request    ->
                        call controller ->   # call Session().  this establishes a new,
                                             # contextual Session.
                                             session = Session()

                                             # load some objects, save some changes
                                             objects = session.query(MyClass).all()

                                             # some other code calls Session, it's the
                                             # same contextual session as "sess"
                                             session2 = Session()
                                             session2.add(foo)
                                             session2.commit()

                                             # generate content to be returned
                                             return generate_content()
                        Session.remove() <-
    web response   <-

The above example illustrates an explicit call to :meth:`.ScopedSession.remove`. This
has the effect such that each web request starts fresh with a brand new
session, and is the most definitive approach to closing out a request.

It's not strictly necessary to remove the session at the end of the request -
other options include calling :meth:`.Session.close`, :meth:`.Session.rollback`,
:meth:`.Session.commit` at the end so that the existing session returns 
its connections to the pool and removes any existing transactional context.
Doing nothing is an option too, if individual controller methods take responsibility
for ensuring that no transactions remain open after a request ends.

Contextual Session API
-----------------------

.. autofunction:: sqlalchemy.orm.scoped_session

.. autoclass:: sqlalchemy.orm.scoping.ScopedSession
   :members:

.. autoclass:: sqlalchemy.util.ScopedRegistry
    :members:

.. autoclass:: sqlalchemy.util.ThreadLocalRegistry

.. _session_partitioning:

Partitioning Strategies
=======================

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()

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`.

Session Utilities
=================

.. autofunction:: make_transient

.. autofunction:: object_session

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`.

.. currentmodule:: sqlalchemy.orm.attributes

.. autofunction:: del_attribute

.. autofunction:: get_attribute

.. autofunction:: get_history

.. autofunction:: init_collection

.. function:: instance_state

    Return the :class:`InstanceState` for a given object.

.. autofunction:: is_instrumented

.. function:: manager_of_class

    Return the :class:`ClassManager` for a given class.

.. autofunction:: set_attribute

.. autofunction:: set_committed_value

.. autoclass:: History
    :members:

.. attribute:: sqlalchemy.orm.attributes.PASSIVE_NO_INITIALIZE

   Symbol indicating that loader callables should
   not be fired off, and a non-initialized attribute 
   should remain that way.

.. attribute:: sqlalchemy.orm.attributes.PASSIVE_NO_FETCH

   Symbol indicating that loader callables should not boe fired off.
   Non-initialized attributes should be initialized to an empty value.

.. attribute:: sqlalchemy.orm.attributes.PASSIVE_OFF

    Symbol indicating that loader callables should be executed.

