.. _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 `_ - 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 :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 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 :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. 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. When you write your application, place the result of the :func:`.sessionmaker` call at the global level. The resulting ``Session`` class, configured for your application, should then be used by the rest of the applcation as the source of new :class:`.Session` instances. An extremely common step taken by applications, including virtually all web applications, is to further wrap the :func:`.sessionmaker` construct in a so-called "contextual" session, provided by the :func:`.scoped_session` construct. This construct places the :func:`.sessionmaker` into a **registry** that maintains a single :class:`.Session` per application thread. Information on using contextual sessions is at :ref:`unitofwork_contextual`. Adding additional configuration to an Existing sessionmaker() -------------------------------------------------------------- A common scenario is where the :func:`.sessionmaker` is invoked at module import time, however the generation of one or more :class:`.Engine` instances to be associated with the :func:`.sessionmaker` has not yet proceeded. For this use case, the :func:`.sessionmaker` construct offers the :meth:`.sessionmaker.configure` method, which will place additional configuration directives into an existing :func:`.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 :func:`.sessionmaker` construct's class itself. 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:`~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 `_, 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
with identity key (, (1,)) conflicts with persistent instance
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': , '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 :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 `_. 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, cascade="all, delete-orphan"), 'customer' : relationship(User, secondary=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, 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, 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 as of 0.6.5 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. .. _unitofwork_transaction: 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 .. _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. 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' .. _session_subtransactions: 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.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.