Release: 1.3.2 | Release Date: April 2, 2019

# SQLAlchemy 1.3 Documentation

# Deprecated ORM Event Interfaces

This section describes the class-based ORM event interface which first existed in SQLAlchemy 0.1, which progressed with more kinds of events up until SQLAlchemy 0.5. The non-ORM analogue is described at Deprecated Event Interfaces.

Deprecated since version 0.7: As of SQLAlchemy 0.7, the new event system described in Events replaces the extension/proxy/listener system, providing a consistent interface to all events without the need for subclassing.

## Mapper Events

class `sqlalchemy.orm.interfaces.``MapperExtension`

  Base implementation for `Mapper` event hooks.

  Deprecated since version 0.7: `MapperExtension` is deprecated and will be removed in a future release. Please refer to `event.listen()` in conjunction with the `MapperEvents` listener interface.

  New extension classes subclass `MapperExtension` and are specified using the `extension` mapper() argument, which is a single `MapperExtension` or a list of such:

      from sqlalchemy.orm.interfaces import MapperExtension

      class MyExtension(MapperExtension):
          def before_insert(self, mapper, connection, instance):
              print "instance %s before insert !" % instance

      m = mapper(User, users_table, extension=MyExtension())

  A single mapper can maintain a chain of `MapperExtension` objects. When a particular mapping event occurs, the corresponding method on each `MapperExtension` is invoked serially, and each method has the ability to halt the chain from proceeding further:

      m = mapper(User, users_table, extension=[ext1, ext2, ext3])

  Each `MapperExtension` method returns the symbol EXT_CONTINUE by default. This symbol generally means “move to the next `MapperExtension` for processing”. For methods that return objects like translated rows or new object instances, EXT_CONTINUE means the result of the method should be ignored. In some cases it’s required for a default mapper activity to be performed, such as adding a new instance to a result list.

  The symbol EXT_STOP has significance within a chain of `MapperExtension` objects that the chain will be stopped when this symbol is returned. Like EXT_CONTINUE, it also has additional significance in some cases that a default mapper activity will not be performed.

  `after_delete`(mapper, connection, instance)

    Receive an object instance after that instance is deleted.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

  `after_insert`(mapper, connection, instance)

    Receive an object instance after that instance is inserted.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

  `after_update`(mapper, connection, instance)

    Receive an object instance after that instance is updated.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

  `before_delete`(mapper, connection, instance)

    Receive an object instance before that instance is deleted.

    Note that no changes to the overall flush plan can be made here; and manipulation of the `Session` will not have the desired effect. To manipulate the `Session` within an extension, use `SessionExtension`.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

  `before_insert`(mapper, connection, instance)

    Receive an object instance before that instance is inserted into its table.

    This is a good place to set up primary key values and such that aren’t handled otherwise.

    Column-based attributes can be modified within this method which will result in the new value being inserted. However no changes to the overall flush plan can be made, and manipulation of the `Session` will not have the desired effect. To manipulate the `Session` within an extension, use `SessionExtension`.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

  `before_update`(mapper, connection, instance)

    Receive an object instance before that instance is updated.

    Note that this method is called for all instances that are marked as “dirty”, even those which have no net changes to their column-based attributes. An object is marked as dirty when any of its column-based attributes have a “set attribute” operation called or when any of its collections are modified. If, at update time, no column-based attributes have any net changes, no UPDATE statement will be issued. This means that an instance being sent to before_update is not a guarantee that an UPDATE statement will be issued (although you can affect the outcome here).

    To detect if the column-based attributes on the object have net changes, and will therefore generate an UPDATE statement, use `object_session(instance).is_modified(instance, include_collections=False)`.

    Column-based attributes can be modified within this method which will result in the new value being updated. However no changes to the overall flush plan can be made, and manipulation of the `Session` will not have the desired effect. To manipulate the `Session` within an extension, use `SessionExtension`.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

  `init_failed`(mapper, class_, oldinit, instance, args, kwargs)

    Receive an instance when its constructor has been called, and raised an exception.

    This method is only called during a userland construction of an object. It is not called when an object is loaded from the database.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

  `init_instance`(mapper, class_, oldinit, instance, args, kwargs)

    Receive an instance when its constructor is called.

    This method is only called during a userland construction of an object. It is not called when an object is loaded from the database.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

  `instrument_class`(mapper, class_)

    Receive a class when the mapper is first constructed, and has applied instrumentation to the mapped class.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

  `reconstruct_instance`(mapper, instance)

    Receive an object instance after it has been created via `__new__`, and after initial attribute population has occurred.

    This typically occurs when the instance is created based on incoming result rows, and is only called once for that instance’s lifetime.

    Note that during a result-row load, this method is called upon the first row received for this instance. Note that some attributes and collections may or may not be loaded or even initialized, depending on what’s present in the result rows.

    The return value is only significant within the `MapperExtension` chain; the parent mapper’s behavior isn’t modified by this method.

## Session Events

class `sqlalchemy.orm.interfaces.``SessionExtension`

  Base implementation for `Session` event hooks.

  Deprecated since version 0.7: `SessionExtension` is deprecated and will be removed in a future release. Please refer to `event.listen()` in conjunction with the `SessionEvents` listener interface.

  Subclasses may be installed into a `Session` (or `sessionmaker`) using the `extension` keyword argument:

      from sqlalchemy.orm.interfaces import SessionExtension

      class MySessionExtension(SessionExtension):
          def before_commit(self, session):
              print "before commit!"

      Session = sessionmaker(extension=MySessionExtension())

  The same `SessionExtension` instance can be used with any number of sessions.

  `after_attach`(session, instance)

    Execute after an instance is attached to a session.

    This is called after an add, delete or merge.

  `after_begin`(session, transaction, connection)

    Execute after a transaction is begun on a connection

    transaction is the SessionTransaction. This method is called after an engine level transaction is begun on a connection.

  `after_bulk_delete`(session, query, query_context, result)

    Execute after a bulk delete operation to the session.

    This is called after a session.query(…).delete()

    query is the query object that this delete operation was called on. query_context was the query context object. result is the result object returned from the bulk operation.

  `after_bulk_update`(session, query, query_context, result)

    Execute after a bulk update operation to the session.

    This is called after a session.query(…).update()

    query is the query object that this update operation was called on. query_context was the query context object. result is the result object returned from the bulk operation.

  `after_commit`(session)

    Execute after a commit has occurred.

    Note that this may not be per-flush if a longer running transaction is ongoing.

  `after_flush`(session, flush_context)

    Execute after flush has completed, but before commit has been called.

    Note that the session’s state is still in pre-flush, i.e. ‘new’, ‘dirty’, and ‘deleted’ lists still show pre-flush state as well as the history settings on instance attributes.

  `after_flush_postexec`(session, flush_context)

    Execute after flush has completed, and after the post-exec state occurs.

    This will be when the ‘new’, ‘dirty’, and ‘deleted’ lists are in their final state. An actual commit() may or may not have occurred, depending on whether or not the flush started its own transaction or participated in a larger transaction.

  `after_rollback`(session)

    Execute after a rollback has occurred.

    Note that this may not be per-flush if a longer running transaction is ongoing.

  `before_commit`(session)

    Execute right before commit is called.

    Note that this may not be per-flush if a longer running transaction is ongoing.

  `before_flush`(session, flush_context, instances)

    Execute before flush process has started.

    instances is an optional list of objects which were passed to the `flush()` method.

## Attribute Events

class `sqlalchemy.orm.interfaces.``AttributeExtension`

  Base implementation for `AttributeImpl` event hooks, events that fire upon attribute mutations in user code.

  Deprecated since version 0.7: `AttributeExtension` is deprecated and will be removed in a future release. Please refer to `event.listen()` in conjunction with the `AttributeEvents` listener interface.

  `AttributeExtension` is used to listen for set, remove, and append events on individual mapped attributes. It is established on an individual mapped attribute using the extension argument, available on `column_property()`, `relationship()`, and others:

      from sqlalchemy.orm.interfaces import AttributeExtension
      from sqlalchemy.orm import mapper, relationship, column_property

      class MyAttrExt(AttributeExtension):
          def append(self, state, value, initiator):
              print "append event !"
              return value

          def set(self, state, value, oldvalue, initiator):
              print "set event !"
              return value

      mapper(SomeClass, sometable, properties={
          'foo':column_property(sometable.c.foo, extension=MyAttrExt()),
          'bar':relationship(Bar, extension=MyAttrExt())
      })

  Note that the `AttributeExtension` methods `append()` and `set()` need to return the `value` parameter. The returned value is used as the effective value, and allows the extension to change what is ultimately persisted.

  AttributeExtension is assembled within the descriptors associated with a mapped class.

  `active_history` = True

    indicates that the set() method would like to receive the ‘old’ value, even if it means firing lazy callables.

    Note that `active_history` can also be set directly via `column_property()` and `relationship()`.

  `append`(state, value, initiator)

    Receive a collection append event.

    The returned value will be used as the actual value to be appended.

  `remove`(state, value, initiator)

    Receive a remove event.

    No return value is defined.

  `set`(state, value, oldvalue, initiator)

    Receive a set event.

    The returned value will be used as the actual value to be set.