# SQLAlchemy 0.6.5 Documentation

Search:
Version: 0.6.5 Last Updated: 10/24/2010 13:31:44

# Using the Session

The `orm.mapper()` function and `declarative` extensions are the primary configurational interface for the ORM. Once mappings are configured, the primary usage interface for persistence operations is the `Session`.

## What does the Session do ?

In the most general sense, the `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 `Query` object, which sends queries to the database using the `Session` object’s current database connection, populating result rows into objects that are then stored in the `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 `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 `Engine` that is associated either with the `Session` itself or with the mapped `Table` objects being operated upon. This connection represents an ongoing transaction, which remains in effect until the `Session` is instructed to commit or roll back its pending state.

All changes to objects maintained by a `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 `Session`, it’s important to note that the objects which are associated with it are proxy objects to the transaction being held by the `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 `Session`, and to continue using them, though this practice has its caveats. It’s intended that usually, you’d re-associate detached objects another `Session` when you want to work with them again, so that they can resume their normal task of representing database state.

## Getting a Session

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

The usage of `sessionmaker()` is illustrated below:

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

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

A typical setup will associate the `sessionmaker()` with an `Engine`, so that each `Session` generated will use this `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 `Engine` with an existing `sessionmaker()` using the `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 `Session` objects with an `Engine` on each invocation:

    session = Session(bind=engine)

...or directly with a `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 Joining a Session into an 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 `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 `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 `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 `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 `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 `Session` at the “class” level to the engine later on, using `configure()`.

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

- When do I make a `Session` ?

  > You typically invoke `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 `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 `Session` using `Session.add()` or `Session.merge()` before working with the object (or in the case of merge, its state) again.

  > It is also very common that a `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 `Session` per thread, `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 `Session` -per-thread, `Session` -per-set-of-objects patterns, the `scoped_session()` function is provided which produces a thread-managed registry of `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 Contextual/Thread-local Sessions.

- 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 `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 `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 Beaker Caching example.

- How can I get the `Session` for a certain object ?

  > Use the `object_session()` classmethod available on `Session`:

  >     session = Session.object_session(someobject)

- 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 `scoped_session()` to transparently manage sessions per thread. More on this at Contextual/Thread-local Sessions.

### Querying

The `query()` function takes one or more entities and returns a new `Query` object which will issue mapper queries within the context of this Session. An entity is defined as a mapped class, a `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 `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 `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 `Query` object is introduced in great detail in Object Relational Tutorial, and further documented in Querying.

### Adding New or Existing Items

`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 `add_all()`:

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

The `add()` operation cascades along the `save-update` cascade. For more details see the section Cascades.

### Merging

`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 `merge()`, the given instance is not placed within the session, and can be associated with a different session or detached. `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 `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 `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.

`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 `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`, `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

`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 `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 `merge()` operation essentially does nothing. Cascade can be configured via the `cascade` option on `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 `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 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 `merge()` issues can be examined by first checking - is the object prematurely in the session ?

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

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

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

### Deleting

The `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 `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 `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 `delete()` for more details.

### Flushing

When the `Session` is used with its default configuration, the flush step is nearly always done transparently. Specifically, the flush occurs before any individual `Query` is issued, as well as within the `commit()` call before the transaction is committed. It also occurs before a SAVEPOINT is issued when `begin_nested()` is used.

Regardless of the autoflush setting, a flush can always be forced by issuing `flush()`:

    session.flush()

The “flush-on-Query” aspect of the behavior can be disabled by constructing `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 `Session` has been configured with `autocommit=True`, a setting that disables the session’s persistent transactional state. If no transaction is present, `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 `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

`commit()` is used to commit the current transaction. It always issues `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 `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 `begin()` method.

Another behavior of `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 `Query` result set, they receive the most recent state. To disable this behavior, configure `sessionmaker()` with `expire_on_commit=False`.

Normally, instances loaded into the `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 `Session` behaves in exactly the same way with regard to attribute state, except no transaction is present.

### Rolling Back

`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 `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 `Session` may safely continue usage after a rollback occurs.

When a `flush()` fails, typically for reasons like primary key, foreign key, or “not nullable” constraint violations, a `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 `Session`. What it means here is that even though the database transaction has been rolled back, the end user must still issue `rollback()` to fully reset the state of the `Session`.

### Expunging

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

    session.expunge(obj1)

To remove all items, call `expunge_all()` (this method was formerly known as `clear()`).

### Closing

The `close()` method issues a `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 `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 `expire()` and `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 `Query.get()`, so any relationships configured with eager loading should also load within the scope of the refresh operation.

`refresh()` and `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 `expire_all()`:

    session.expire_all()

Note that `expire_all()` is called automatically whenever `commit()` or `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 `expire_all()` directly - plenty of state will remain on the current transaction until it is rolled back or committed or otherwise removed.

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

### Session Attributes

The `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 `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 `sessionmaker()` with the `weak_identity_map=False` setting.

## Cascades

Mappers support the concept of configurable cascade behavior on `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 `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 `add()` operation. `save-update` cascade indicates that an `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 `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 `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 `add()` `i1` to the session at a later point. This option may be helpful for situations where an object needs to be kept out of a session until it’s construction is completed, but still needs to be given associations to objects which are already persistent in the target session.

## Managing Transactions

The `Session` manages transactions across all engines associated with it. As the `Session` receives requests to execute SQL statements using a particular `Engine` or `Connection`, it adds each individual `Engine` encountered to its transactional state and maintains an open connection for each one (note that a simple application normally has just one `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 `rollback()` or `commit()` methods. After either is called, the `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 `begin()`. With an `autocommit=True` session that’s been placed into a transaction using `begin()`, the session releases all connection resources after a `commit()` or `rollback()` and remains transaction-less (with the exception of flushes) until the next `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 `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'

### Using SAVEPOINT

SAVEPOINT transactions, if supported by the underlying engine, may be delineated using the `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

`begin_nested()` may be called any number of times, which will issue a new SAVEPOINT with a unique identifier for each call. For each `begin_nested()` call, a corresponding `rollback()` or `commit()` must be issued.

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

### 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 `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 `Session` within its transactional context. This is most easily accomplished using the `execute()` method, which returns a `ResultProxy` in the same manner as an `Engine` or `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 `Connection` held by the `Session` is accessible using the `connection()` method:

    connection = session.connection()

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

    Session = sessionmaker()
    session = Session()

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

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

    connection = session.connection(MyMappedClass)

## Joining a Session into an External Transaction

If a `Connection` is being used which is already in a transactional state (i.e. has a `Transaction` established), a `Session` can be made to participate within that transaction by just binding the `Session` to that `Connection`. The usual rationale for this is a test suite that allows ORM code to work freely with a `Session`, including the ability to call `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 `Session.commit()` as well as `Transaction.rollback()`. This is an example of where we take advantage of the `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 `Session` object and `sessionmaker()` function

`sqlalchemy.orm.session.``sessionmaker`(bind=None, class_=None, autoflush=True, autocommit=False, expire_on_commit=True, **kwargs)

  Generate a custom-configured `Session` class.

  The returned object is a subclass of `Session`, which, when instantiated with no arguments, uses the keyword arguments configured here as its constructor arguments.

  It is intended that the sessionmaker() function be called within the global scope of an application, and the returned class be made available to the rest of the application as the single class used to instantiate sessions.

  e.g.:

      # global scope
      Session = sessionmaker(autoflush=False)

      # later, in a local scope, create and use a session:
      sess = Session()

  Any keyword arguments sent to the constructor itself will override the “configured” keywords:

      Session = sessionmaker()

      # bind an individual session to a connection
      sess = Session(bind=connection)

  The class also includes a special classmethod `configure()`, which allows additional configurational options to take place after the custom `Session` class has been generated. This is useful particularly for defining the specific `Engine` (or engines) to which new instances of `Session` should be bound:

      Session = sessionmaker()
      Session.configure(bind=create_engine('sqlite:///foo.db'))

      sess = Session()

  Options:

  Parameters:

  - autocommit –

    Defaults to `False`. When `True`, the `Session` does not keep a persistent transaction running, and will acquire connections from the engine on an as-needed basis, returning them immediately after their use. Flushes will begin and commit (or possibly rollback) their own transaction if no transaction is present. When using this mode, the session.begin() method may be used to begin a transaction explicitly.

    Leaving it on its default value of `False` means that the `Session` will acquire a connection and begin a transaction the first time it is used, which it will maintain persistently until `rollback()`, `commit()`, or `close()` is called. When the transaction is released by any of these methods, the `Session` is ready for the next usage, which will again acquire and maintain a new connection/transaction.

  - autoflush – When `True`, all query operations will issue a `flush()` call to this `Session` before proceeding. This is a convenience feature so that `flush()` need not be called repeatedly in order for database queries to retrieve results. It’s typical that `autoflush` is used in conjunction with `autocommit=False`. In this scenario, explicit calls to `flush()` are rarely needed; you usually only need to call `commit()` (which flushes) to finalize changes.
  - bind – An optional `Engine` or `Connection` to which this `Session` should be bound. When specified, all SQL operations performed by this session will execute via this connectable.
  - binds –
    An optional dictionary which contains more granular “bind”
      information than the `bind` parameter provides. This dictionary can map individual `Table` instances as well as `Mapper` instances to individual `Engine` or `Connection` objects. Operations which proceed relative to a particular `Mapper` will consult this dictionary for the direct `Mapper` instance as well as the mapper’s `mapped_table` attribute in order to locate an connectable to use. The full resolution is described in the `get_bind()` method of `Session`. Usage looks like:

          Session = sessionmaker(binds={
              SomeMappedClass: create_engine('postgresql://engine1'),
              somemapper: create_engine('postgresql://engine2'),
              some_table: create_engine('postgresql://engine3'),
              })

    Also see the `Session.bind_mapper()` and `Session.bind_table()` methods.

  - class_ – Specify an alternate class other than `sqlalchemy.orm.session.Session` which should be used by the returned class. This is the only argument that is local to the `sessionmaker()` function, and is not sent directly to the constructor for `Session`.
  - _enable_transaction_accounting – Defaults to `True`. A legacy-only flag which when `False` disables all 0.5-style object accounting on transaction boundaries, including auto-expiry of instances on rollback and commit, maintenance of the “new” and “deleted” lists upon rollback, and autoflush of pending changes upon begin(), all of which are interdependent.
  - expire_on_commit – Defaults to `True`. When `True`, all instances will be fully expired after each `commit()`, so that all attribute/object access subsequent to a completed transaction will load from the most recent database state.
  - extension – An optional `SessionExtension` instance, or a list of such instances, which will receive pre- and post- commit and flush events, as well as a post-rollback event. User- defined code may be placed within these hooks using a user-defined subclass of `SessionExtension`.
  - query_cls – Class which should be used to create new Query objects, as returned by the `query()` method. Defaults to `Query`.
  - twophase – When `True`, all transactions will be started as a “two phase” transaction, i.e. using the “two phase” semantics of the database in use along with an XID. During a `commit()`, after `flush()` has been issued for all attached databases, the `prepare()` method on each database’s `TwoPhaseTransaction` will be called. This allows each database to roll back the entire transaction, before each transaction is committed.
  - weak_identity_map – When set to the default value of `True`, a weak-referencing map is used; instances which are not externally referenced will be garbage collected immediately. For dereferenced instances which have pending changes present, the attribute management system will create a temporary strong-reference to the object which lasts until the changes are flushed to the database, at which point it’s again dereferenced. Alternatively, when using the value `False`, the identity map uses a regular Python dictionary to store instances. The session will maintain all instances present until they are removed using expunge(), clear(), or purge().

class `sqlalchemy.orm.session.``Session`(bind=None, autoflush=True, expire_on_commit=True, _enable_transaction_accounting=True, autocommit=False, twophase=False, weak_identity_map=True, binds=None, extension=None, query_cls=<class 'sqlalchemy.orm.query.Query'>)

  Manages persistence operations for ORM-mapped objects.

  The Session’s usage paradigm is described at Using the Session.

  `__init__`(bind=None, autoflush=True, expire_on_commit=True, _enable_transaction_accounting=True, autocommit=False, twophase=False, weak_identity_map=True, binds=None, extension=None, query_cls=<class 'sqlalchemy.orm.query.Query'>)

    Construct a new Session.

    Arguments to `Session` are described using the `sessionmaker()` function, which is the typical point of entry.

  `add`(instance)

    Place an object in the `Session`.

    Its state will be persisted to the database on the next flush operation.

    Repeated calls to `add()` will be ignored. The opposite of `add()` is `expunge()`.

  `add_all`(instances)

    Add the given collection of instances to this `Session`.

  `begin`(subtransactions=False, nested=False)

    Begin a transaction on this Session.

    If this Session is already within a transaction, either a plain transaction or nested transaction, an error is raised, unless `subtransactions=True` or `nested=True` is specified.

    The `subtransactions=True` flag indicates that this `begin()` can create a subtransaction if a transaction is already in progress. A subtransaction is a non-transactional, delimiting construct that allows matching begin()/commit() pairs to be nested together, with only the outermost begin/commit pair actually affecting transactional state. When a rollback is issued, the subtransaction will directly roll back the innermost real transaction, however each subtransaction still must be explicitly rolled back to maintain proper stacking of subtransactions.

    If no transaction is in progress, then a real transaction is begun.

    The `nested` flag begins a SAVEPOINT transaction and is equivalent to calling `begin_nested()`.

  `begin_nested`()

    Begin a nested transaction on this Session.

    The target database(s) must support SQL SAVEPOINTs or a SQLAlchemy-supported vendor implementation of the idea.

    The nested transaction is a real transation, unlike a “subtransaction” which corresponds to multiple `begin()` calls. The next `rollback()` or `commit()` call will operate upon this nested transaction.

  `bind_mapper`(mapper, bind)

    Bind operations for a mapper to a Connectable.

    mapper
      A mapper instance or mapped class
    bind
      Any Connectable: a `Engine` or `Connection`.

    All subsequent operations involving this mapper will use the given bind.

  `bind_table`(table, bind)

    Bind operations on a Table to a Connectable.

    table
      A `Table` instance
    bind
      Any Connectable: a `Engine` or `Connection`.

    All subsequent operations involving this `Table` will use the given bind.

  `close`()

    Close this Session.

    This clears all items and ends any transaction in progress.

    If this session were created with `autocommit=False`, a new transaction is immediately begun. Note that this new transaction does not use any connection resources until they are first needed.

  classmethod `close_all`()

    Close all sessions in memory.

  `commit`()

    Flush pending changes and commit the current transaction.

    If no transaction is in progress, this method raises an InvalidRequestError.

    By default, the `Session` also expires all database loaded state on all ORM-managed attributes after transaction commit. This so that subsequent operations load the most recent data from the database. This behavior can be disabled using the `expire_on_commit=False` option to `sessionmaker()` or the `Session` constructor.

    If a subtransaction is in effect (which occurs when begin() is called multiple times), the subtransaction will be closed, and the next call to `commit()` will operate on the enclosing transaction.

    For a session configured with autocommit=False, a new transaction will be begun immediately after the commit, but note that the newly begun transaction does not use any connection resources until the first SQL is actually emitted.

  `connection`(mapper=None, clause=None)

    Return the active Connection.

    Retrieves the `Connection` managing the current transaction. Any operations executed on the Connection will take place in the same transactional context as `Session` operations.

    For `autocommit` Sessions with no active manual transaction, `connection()` is a passthrough to `contextual_connect()` on the underlying engine.

    Ambiguity in multi-bind or unbound Sessions can be resolved through any of the optional keyword arguments. See `get_bind()` for more information.

    mapper
      Optional, a `mapper` or mapped class
    clause
      Optional, any `ClauseElement`
  `delete`(instance)

    Mark an instance as deleted.

    The database delete operation occurs upon `flush()`.

  `deleted`

    The set of all instances marked as ‘deleted’ within this `Session`

  `dirty`

    The set of all persistent instances considered dirty.

    Instances are considered dirty when they were modified but not deleted.

    Note that this ‘dirty’ calculation is ‘optimistic’; most attribute-setting or collection modification operations will mark an instance as ‘dirty’ and place it in this set, even if there is no net change to the attribute’s value. At flush time, the value of each attribute is compared to its previously saved value, and if there’s no net change, no SQL operation will occur (this is a more expensive operation so it’s only done at flush time).

    To check if an instance has actionable net changes to its attributes, use the is_modified() method.

  `execute`(clause, params=None, mapper=None, **kw)

    Execute a clause within the current transaction.

    Returns a `ResultProxy` representing results of the statement execution, in the same manner as that of an `Engine` or `Connection`.

    `Session.execute()` accepts any executable clause construct, such as `select()`, `insert()`, `update()`, `delete()`, and `text()`, and additionally accepts plain strings that represent SQL statements. If a plain string is passed, it is first converted to a `text()` construct, which here means that bind parameters should be specified using the format `:param`.

    The statement is executed within the current transactional context of this `Session`. If this `Session` is set for “autocommit”, and no transaction is in progress, an ad-hoc transaction will be created for the life of the result (i.e., a connection is checked out from the connection pool, which is returned when the result object is closed).

    If the `Session` is not bound to an `Engine` or `Connection`, the given clause will be inspected for binds (i.e., looking for “bound metadata”). If the session is bound to multiple connectables, the `mapper` keyword argument is typically passed in to specify which bind should be used (since the `Session` keys multiple bind sources to a series of `mapper()` objects). See `get_bind()` for further details on bind resolution.

    Parameters:

    - clause – A ClauseElement (i.e. select(), text(), etc.) or string SQL statement to be executed
    - params – Optional, a dictionary of bind parameters.
    - mapper – Optional, a `mapper` or mapped class
    - **kw – Additional keyword arguments are sent to `get_bind()` which locates a connectable to use for the execution.

  `expire`(instance, attribute_names=None)

    Expire the attributes on an instance.

    Marks the attributes of an instance as out of date. When an expired attribute is next accessed, a query will be issued to the `Session` object’s current transactional context in order to load all expired attributes for the given instance. Note that a highly isolated transaction will return the same values as were previously read in that same transaction, regardless of changes in database state outside of that transaction.

    To expire all objects in the `Session` simultaneously, use `Session.expire_all()`.

    The `Session` object’s default behavior is to expire all state whenever the `Session.rollback()` or `Session.commit()` methods are called, so that new state can be loaded for the new transaction. For this reason, calling `Session.expire()` only makes sense for the specific case that a non-ORM SQL statement was emitted in the current transaction.

    Parameters:

    - instance – The instance to be refreshed.
    - attribute_names – optional list of string attribute names indicating a subset of attributes to be expired.

  `expire_all`()

    Expires all persistent instances within this Session.

    When any attributes on a persitent instance is next accessed, a query will be issued using the `Session` object’s current transactional context in order to load all expired attributes for the given instance. Note that a highly isolated transaction will return the same values as were previously read in that same transaction, regardless of changes in database state outside of that transaction.

    To expire individual objects and individual attributes on those objects, use `Session.expire()`.

    The `Session` object’s default behavior is to expire all state whenever the `Session.rollback()` or `Session.commit()` methods are called, so that new state can be loaded for the new transaction. For this reason, calling `Session.expire_all()` should not be needed when autocommit is `False`, assuming the transaction is isolated.

  `expunge`(instance)

    Remove the instance from this `Session`.

    This will free all internal references to the instance. Cascading will be applied according to the expunge cascade rule.

  `expunge_all`()

    Remove all object instances from this `Session`.

    This is equivalent to calling `expunge(obj)` on all objects in this `Session`.

  `flush`(objects=None)

    Flush all the object changes to the database.

    Writes out all pending object creations, deletions and modifications to the database as INSERTs, DELETEs, UPDATEs, etc. Operations are automatically ordered by the Session’s unit of work dependency solver..

    Database operations will be issued in the current transactional context and do not affect the state of the transaction. You may flush() as often as you like within a transaction to move changes from Python to the database’s transaction buffer.

    For `autocommit` Sessions with no active manual transaction, flush() will create a transaction on the fly that surrounds the entire set of operations int the flush.

    objects
      Optional; a list or tuple collection. Restricts the flush operation to only these objects, rather than all pending changes. Deprecated - this flag prevents the session from properly maintaining accounting among inter-object relations and can cause invalid results.
  `get_bind`(mapper, clause=None)

    Return an engine corresponding to the given arguments.

    All arguments are optional.

    mapper
      Optional, a `Mapper` or mapped class
    clause
      Optional, A ClauseElement (i.e. select(), text(), etc.)
  `is_active`

    True if this Session has an active transaction.

  `is_modified`(instance, include_collections=True, passive=False)

    Return `True` if instance has modified attributes.

    This method retrieves a history instance for each instrumented attribute on the instance and performs a comparison of the current value to its previously committed value.

    `include_collections` indicates if multivalued collections should be included in the operation. Setting this to False is a way to detect only local-column based properties (i.e. scalar columns or many-to-one foreign keys) that would result in an UPDATE for this instance upon flush.

    The `passive` flag indicates if unloaded attributes and collections should not be loaded in the course of performing this test.

    A few caveats to this method apply:

    - Instances present in the ‘dirty’ collection may result in a value of `False` when tested with this method. This because while the object may have received attribute set events, there may be no net changes on its state.

    - Scalar attributes may not have recorded the “previously” set value when a new value was applied, if the attribute was not loaded, or was expired, at the time the new value was received - in these cases, the attribute is assumed to have a change, even if there is ultimately no net change against its database value. SQLAlchemy in most cases does not need the “old” value when a set event occurs, so it skips the expense of a SQL call if the old value isn’t present, based on the assumption that an UPDATE of the scalar value is usually needed, and in those few cases where it isn’t, is less expensive on average than issuing a defensive SELECT.

      The “old” value is fetched unconditionally only if the attribute container has the “active_history” flag set to `True`. This flag is set typically for primary key attributes and scalar references that are not a simple many-to-one.

  `merge`(instance, load=True, **kw)

    Copy the state an instance onto the persistent instance with the same identifier.

    If there is no persistent instance currently associated with the session, it will be loaded. Return the persistent instance. If the given instance is unsaved, save a copy of and return it as a newly persistent instance. The given instance does not become associated with the session.

    This operation cascades to associated instances if the association is mapped with `cascade="merge"`.

    See Merging for a detailed discussion of merging.

  `new`

    The set of all instances marked as ‘new’ within this `Session`.

  classmethod `object_session`(instance)

    Return the `Session` to which an object belongs.

  `prepare`()

    Prepare the current transaction in progress for two phase commit.

    If no transaction is in progress, this method raises an InvalidRequestError.

    Only root transactions of two phase sessions can be prepared. If the current transaction is not such, an InvalidRequestError is raised.

  `prune`()

    Remove unreferenced instances cached in the identity map.

    Note that this method is only meaningful if “weak_identity_map” is set to False. The default weak identity map is self-pruning.

    Removes any object in this Session’s identity map that is not referenced in user code, modified, new or scheduled for deletion. Returns the number of objects pruned.

  `query`(*entities, **kwargs)

    Return a new `Query` object corresponding to this `Session`.

  `refresh`(instance, attribute_names=None, lockmode=None)

    Expire and refresh the attributes on the given instance.

    A query will be issued to the database and all attributes will be refreshed with their current database value.

    Lazy-loaded relational attributes will remain lazily loaded, so that the instance-wide refresh operation will be followed immediately by the lazy load of that attribute.

    Eagerly-loaded relational attributes will eagerly load within the single refresh operation.

    Note that a highly isolated transaction will return the same values as were previously read in that same transaction, regardless of changes in database state outside of that transaction - usage of `refresh()` usually only makes sense if non-ORM SQL statement were emitted in the ongoing transaction, or if autocommit mode is turned on.

    Parameters:

    - attribute_names – optional. An iterable collection of string attribute names indicating a subset of attributes to be refreshed.
    - lockmode – Passed to the `Query` as used by `with_lockmode()`.

  `rollback`()

    Rollback the current transaction in progress.

    If no transaction is in progress, this method is a pass-through.

    This method rolls back the current transaction or nested transaction regardless of subtransactions being in effect. All subtransactions up to the first real transaction are closed. Subtransactions occur when begin() is called multiple times.

  `scalar`(clause, params=None, mapper=None, **kw)

    Like execute() but return a scalar result.

## Contextual/Thread-local Sessions

A common need in applications, particularly those built around web frameworks, is the ability to “share” a `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 `Session` class generated by `sessionmaker()` to provide auto-contextualizing support. This means that whenever you create a `Session` instance with its constructor, you get an existing `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 `scoped_session()` function wraps around the `sessionmaker()` function, and produces an object which behaves the same as the `Session` subclass returned by `sessionmaker()`:

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

However, when you instantiate this `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 `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 `Session()` constructor now returns the same `Session` object every time within the current thread, the object returned by `scoped_session()` also implements most of the `Session` methods and properties at the “class” level, such that you don’t even need to instantiate `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 `Session` for the current thread.

### 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 `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 `Session.close()`, `Session.rollback()`, `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

`sqlalchemy.orm.``scoped_session`(session_factory, scopefunc=None)

  Provides thread-local or scoped management of `Session` objects.

  This is a front-end function to `ScopedSession`.

  Parameters:

  - session_factory – a callable function that produces `Session` instances, such as `sessionmaker()`.
  - scopefunc – Optional “scope” function which would be passed to the `ScopedRegistry`. If None, the `ThreadLocalRegistry` is used by default.

  Returns: | an `ScopedSession` instance

  Usage:

      Session = scoped_session(sessionmaker(autoflush=True))

  To instantiate a Session object which is part of the scoped context, instantiate normally:

      session = Session()

  Most session methods are available as classmethods from the scoped session:

      Session.commit()
      Session.close()

class `sqlalchemy.orm.scoping.``ScopedSession`(session_factory, scopefunc=None)

  Provides thread-local management of Sessions.

  Usage:

      Session = scoped_session(sessionmaker())

  ... use Session normally.

  The internal registry is accessible as well, and by default is an instance of `ThreadLocalRegistry`.

  `__init__`(session_factory, scopefunc=None)
  `configure`(**kwargs)

    reconfigure the sessionmaker used by this ScopedSession.

  `mapper`(*args, **kwargs)

    return a `mapper()` function which associates this ScopedSession with the Mapper.

    Deprecated since version 0.5: `ScopedSession.mapper()` is deprecated. Please see http://www.sqlalchemy.org/trac/wiki/UsageRecipes/SessionAwareMapper for information on how to replicate its behavior.

  `query_property`(query_cls=None)

    return a class property which produces a Query object against the class when called.

    e.g.:

        Session = scoped_session(sessionmaker())

        class MyClass(object):
            query = Session.query_property()

        # after mappers are defined
        result = MyClass.query.filter(MyClass.name=='foo').all()

    Produces instances of the session’s configured query class by default. To override and use a custom implementation, provide a `query_cls` callable. The callable will be invoked with the class’s mapper as a positional argument and a session keyword argument.

    There is no limit to the number of query properties placed on a class.

  `remove`()

    Dispose of the current contextual session.

class `sqlalchemy.util.``ScopedRegistry`(createfunc, scopefunc)

  A Registry that can store one or multiple instances of a single class on the basis of a “scope” function.

  The object implements `__call__` as the “getter”, so by calling `myregistry()` the contained object is returned for the current scope.

  Parameters:

  - createfunc – a callable that returns a new object to be placed in the registry
  - scopefunc – a callable that will return a key to store/retrieve an object.

  `__init__`(createfunc, scopefunc)

    Construct a new `ScopedRegistry`.

    Parameters:

    - createfunc – A creation function that will generate a new value for the current scope, if none is present.
    - scopefunc – A function that returns a hashable token representing the current scope (such as, current thread identifier).

  `clear`()

    Clear the current scope, if any.

  `has`()

    Return True if an object is present in the current scope.

  `set`(obj)

    Set the value forthe current scope.

class `sqlalchemy.util.``ThreadLocalRegistry`(createfunc)

  A `ScopedRegistry` that uses a `threading.local()` variable for storage.

## 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: Horizontal Sharding.

## Session Utilities

`sqlalchemy.orm.session.``make_transient`(instance)

  Make the given instance ‘transient’.

  This will remove its association with any session and additionally will remove its “identity key”, such that it’s as though the object were newly constructed, except retaining its values. It also resets the “deleted” flag on the state if this object had been explicitly deleted by its session.

  Attributes which were “expired” or deferred at the instance level are reverted to undefined, and will not trigger any loads.

`sqlalchemy.orm.session.``object_session`(instance)

  Return the `Session` to which instance belongs.

  If the instance is not a mapped instance, an error is raised.

## 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 ORM Event Interfaces.

`sqlalchemy.orm.attributes.``del_attribute`(instance, key)

  Delete the value of an attribute, firing history events.

  This function may be used regardless of instrumentation applied directly to the class, i.e. no descriptors are required. Custom attribute management schemes will need to make usage of this method to establish attribute state as understood by SQLAlchemy.

`sqlalchemy.orm.attributes.``get_attribute`(instance, key)

  Get the value of an attribute, firing any callables required.

  This function may be used regardless of instrumentation applied directly to the class, i.e. no descriptors are required. Custom attribute management schemes will need to make usage of this method to make usage of attribute state as understood by SQLAlchemy.

`sqlalchemy.orm.attributes.``get_history`(obj, key, **kwargs)

  Return a `History` record for the given object and attribute key.

  Parameters:

  - obj – an object whose class is instrumented by the attributes package.
  - key – string attribute name.
  - kwargs – Optional keyword arguments currently include the `passive` flag, which indicates if the attribute should be loaded from the database if not already present (`PASSIVE_NO_FETCH`), and if the attribute should be not initialized to a blank value otherwise (`PASSIVE_NO_INITIALIZE`). Default is `PASSIVE_OFF`.

`sqlalchemy.orm.attributes.``init_collection`(obj, key)

  Initialize a collection attribute and return the collection adapter.

  This function is used to provide direct access to collection internals for a previously unloaded attribute. e.g.:

      collection_adapter = init_collection(someobject, 'elements')
      for elem in values:
          collection_adapter.append_without_event(elem)

  For an easier way to do the above, see
    `set_committed_value()`.

  obj is an instrumented object instance. An InstanceState is accepted directly for backwards compatibility but this usage is deprecated.

`sqlalchemy.orm.attributes.``instance_state`()

  Return the `InstanceState` for a given object.

`sqlalchemy.orm.attributes.``is_instrumented`(instance, key)

  Return True if the given attribute on the given instance is instrumented by the attributes package.

  This function may be used regardless of instrumentation applied directly to the class, i.e. no descriptors are required.

`sqlalchemy.orm.attributes.``manager_of_class`()

  Return the `ClassManager` for a given class.

`sqlalchemy.orm.attributes.``set_attribute`(instance, key, value)

  Set the value of an attribute, firing history events.

  This function may be used regardless of instrumentation applied directly to the class, i.e. no descriptors are required. Custom attribute management schemes will need to make usage of this method to establish attribute state as understood by SQLAlchemy.

`sqlalchemy.orm.attributes.``set_committed_value`(instance, key, value)

  Set the value of an attribute with no history events.

  Cancels any previous history present. The value should be a scalar value for scalar-holding attributes, or an iterable for any collection-holding attribute.

  This is the same underlying method used when a lazy loader fires off and loads additional data from the database. In particular, this method can be used by application code which has loaded additional attributes or collections through separate queries, which can then be attached to an instance as though it were part of its original loaded state.

class `sqlalchemy.orm.attributes.``History`

  A 3-tuple of added, unchanged and deleted values, representing the changes which have occured on an instrumented attribute.

  Each tuple member is an iterable sequence.

  `added`

    Return the collection of items added to the attribute (the first tuple element).

  `deleted`

    Return the collection of items that have been removed from the attribute (the third tuple element).

  `empty`()

    Return True if this `History` has no changes and no existing, unchanged state.

  `has_changes`()

    Return True if this `History` has changes.

  `non_added`()

    Return a collection of unchanged + deleted.

  `non_deleted`()

    Return a collection of added + unchanged.

  `sum`()

    Return a collection of added + unchanged + deleted.

  `unchanged`

    Return the collection of items that have not changed on the attribute (the second tuple element).

`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.

`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.

`sqlalchemy.orm.attributes.``PASSIVE_OFF`

  Symbol indicating that loader callables should be executed.