Tracking Object and Session Changes with Events¶
SQLAlchemy features an extensive Event Listening system used throughout the Core and ORM. Within the ORM, there are a wide variety of event listener hooks, which are documented at an API level at ORM Events. This collection of events has grown over the years to include lots of very useful new events as well as some older events that aren’t as relevant as they once were. This section will attempt to introduce the major event hooks and when they might be used.
Persistence Events¶
Probably the most widely used series of events are the “persistence” events, which correspond to the flush process. The flush is where all the decisions are made about pending changes to objects and are then emitted out to the database in the form of INSERT, UPDATE, and DELETE staetments.
before_flush()
¶
The SessionEvents.before_flush()
hook is by far the most generally
useful event to use when an application wants to ensure that
additional persistence changes to the database are made when a flush proceeds.
Use SessionEvents.before_flush()
in order to operate
upon objects to validate their state as well as to compose additional objects
and references before they are persisted. Within this event,
it is safe to manipulate the Session’s state, that is, new objects
can be attached to it, objects can be deleted, and indivual attributes
on objects can be changed freely, and these changes will be pulled into
the flush process when the event hook completes.
The typical SessionEvents.before_flush()
hook will be tasked with
scanning the collections Session.new
, Session.dirty
and
Session.deleted
in order to look for objects
where something will be happening.
For illustrations of SessionEvents.before_flush()
, see
examples such as Versioning with a History Table and
Versioning using Temporal Rows.
after_flush()
¶
The SessionEvents.after_flush()
hook is called after the SQL has been
emitted for a flush process, but before the state of the objects that
were flushed has been altered. That is, you can still inspect
the Session.new
, Session.dirty
and
Session.deleted
collections to see what was just flushed, and
you can also use history tracking features like the ones provided
by AttributeState
to see what changes were just persisted.
In the SessionEvents.after_flush()
event, additional SQL can be emitted
to the database based on what’s observed to have changed.
after_flush_postexec()
¶
SessionEvents.after_flush_postexec()
is called soon after
SessionEvents.after_flush()
, but is invoked after the state of
the objects has been modified to account for the flush that just took place.
The Session.new
, Session.dirty
and
Session.deleted
collections are normally completely empty here.
Use SessionEvents.after_flush_postexec()
to inspect the identity map
for finalized objects and possibly emit additional SQL. In this hook,
there is the ability to make new changes on objects, which means the
Session
will again go into a “dirty” state; the mechanics of the
Session
here will cause it to flush again if new changes
are detected in this hook if the flush were invoked in the context of
Session.commit()
; otherwise, the pending changes will be bundled
as part of the next normal flush. When the hook detects new changes within
a Session.commit()
, a counter ensures that an endless loop in this
regard is stopped after 100 iterations, in the case that an
SessionEvents.after_flush_postexec()
hook continually adds new state to be flushed each time it is called.
Mapper-level Events¶
In addition to the flush-level hooks, there is also a suite of hooks that are more fine-grained, in that they are called on a per-object basis and are broken out based on INSERT, UPDATE or DELETE. These are the mapper persistence hooks, and they too are very popular, however these events need to be approached more cautiously, as they proceed within the context of the flush process that is already ongoing; many operations are not safe to proceed here.
The events are:
MapperEvents.before_insert()
MapperEvents.after_insert()
MapperEvents.before_update()
MapperEvents.after_update()
MapperEvents.before_delete()
MapperEvents.after_delete()
Each event is passed the Mapper
,
the mapped object itself, and the Connection
which is being
used to emit an INSERT, UPDATE or DELETE statement. The appeal of these
events is clear, in that if an application wants to tie some activity to
when a specific type of object is persisted with an INSERT, the hook is
very specific; unlike the SessionEvents.before_flush()
event,
there’s no need to search through collections like Session.new
in order to find targets. However, the flush plan which
represents the full list of every single INSERT, UPDATE, DELETE statement
to be emitted has already been decided when these events are called,
and no changes may be made at this stage. Therefore the only changes that are
even possible to the given objects are upon attributes local to the
object’s row. Any other change to the object or other objects will
impact the state of the Session
, which will fail to function
properly.
Operations that are not supported within these mapper-level persistence events include:
Session.add()
Session.delete()
- Mapped collection append, add, remove, delete, discard, etc.
- Mapped relationship attribute set/del events,
i.e.
someobject.related = someotherobject
The reason the Connection
is passed is that it is encouraged that
simple SQL operations take place here, directly on the Connection
,
such as incrementing counters or inserting extra rows within log tables.
When dealing with the Connection
, it is expected that Core-level
SQL operations will be used; e.g. those described in SQL Expression Language Tutorial.
There are also many per-object operations that don’t need to be handled
within a flush event at all. The most common alternative is to simply
establish additional state along with an object inside its __init__()
method, such as creating additional objects that are to be associated with
the new object. Using validators as described in Simple Validators is
another approach; these functions can intercept changes to attributes and
establish additional state changes on the target object in response to the
attribute change. With both of these approaches, the object is in
the correct state before it ever gets to the flush step.
Object Lifecycle Events¶
Another use case for events is to track the lifecycle of objects. This refers to the states first introduced at Quickie Intro to Object States.
As of SQLAlchemy 1.0, there is no direct event interface for tracking of these states. Events that can be used at the moment to track the state of objects include:
InstanceEvents.init()
InstanceEvents.load()
SessionEvents.before_attach()
SessionEvents.after_attach()
SessionEvents.before_flush()
- by scanning the session’s collectionsSessionEvents.after_flush()
- by scanning the session’s collections
SQLAlchemy 1.1 will introduce a comprehensive event system to track the object persistence states fully and unambiguously.
Transaction Events¶
Transaction events allow an application to be notifed when transaction
boundaries occur at the Session
level as well as when the
Session
changes the transactional state on Connection
objects.
SessionEvents.after_transaction_create()
,SessionEvents.after_transaction_end()
- these events track the logical transaction scopes of theSession
in a way that is not specific to individual database connections. These events are intended to help with integration of transaction-tracking systems such aszope.sqlalchemy
. Use these events when the application needs to align some external scope with the transactional scope of theSession
. These hooks mirror the “nested” transactional behavior of theSession
, in that they track logical “subtransactions” as well as “nested” (e.g. SAVEPOINT) transactions.SessionEvents.before_commit()
,SessionEvents.after_commit()
,SessionEvents.after_begin()
,SessionEvents.after_rollback()
,SessionEvents.after_soft_rollback()
- These events allow tracking of transaction events from the perspective of database connections.SessionEvents.after_begin()
in particular is a per-connection event; aSession
that maintains more than one connection will emit this event for each connection individually as those connections become used within the current transaction. The rollback and commit events then refer to when the DBAPI connections themselves have received rollback or commit instructions directly.
Attribute Change Events¶
The attribute change events allow interception of when specific attributes
on an object are modified. These events include AttributeEvents.set()
,
AttributeEvents.append()
, and AttributeEvents.remove()
. These
events are extremely useful, particularly for per-object validation operations;
however, it is often much more convenient to use a “validator” hook, which
uses these hooks behind the scenes; see Simple Validators for
background on this. The attribute events are also behind the mechanics
of backreferences. An example illustrating use of attribute events
is in Attribute Instrumentation.