Relationship Loading Techniques
This section describes the various categories of events which can be intercepted within the SQLAlchemy ORM.
For non-ORM event documentation, see Core Event Interfaces.
A new version of this API with a significantly more flexible and consistent interface will be available in version 0.7.
To use MapperExtension, make your own subclass of it and just send it off to a mapper:
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())Multiple extensions will be chained together and processed in order; they are specified as a list:
m = mapper(User, users_table, extension=[ext1, ext2, ext3])Base implementation for customizing Mapper behavior.
New extension classes subclass MapperExtension and are specified using the extension mapper() argument, which is a single MapperExtension or a list of such. 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.
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.
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.
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.
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.
Receive an object instance before that instance is appended to a result list.
If this method returns EXT_CONTINUE, result appending will proceed normally. if this method returns any other value or None, result appending will not proceed for this instance, giving this extension an opportunity to do the appending itself, if desired.
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.
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.
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.
Receive a row when a new object instance is about to be created from that row.
The method can choose to create the instance itself, or it can return EXT_CONTINUE to indicate normal object creation should take place.
Receive an instance when it’s 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.
Receive an instance when it’s 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.
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.
Receive an instance before that instance has its attributes populated.
This usually corresponds to a newly loaded instance but may also correspond to an already-loaded instance which has unloaded attributes to be populated. The method may be called many times for a single instance, as multiple result rows are used to populate eagerly loaded collections.
If this method returns EXT_CONTINUE, instance population will proceed normally. If any other value or None is returned, instance population will not proceed, giving this extension an opportunity to populate the instance itself, if desired.
As of 0.5, most usages of this hook are obsolete. For a generic “object has been newly created from a row” hook, use reconstruct_instance(), or the @orm.reconstructor decorator.
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.
Perform pre-processing on the given result row and return a new row instance.
This is called when the mapper first receives a row, before the object identity or the instance itself has been derived from that row. The given row may or may not be a RowProxy object - it will always be a dictionary-like object which contains mapped columns as keys. The returned object should also be a dictionary-like object which recognizes mapped columns as keys.
If the ultimate return value is EXT_CONTINUE, the row is not translated.
The SessionExtension applies plugin points for Session objects:
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.
An extension hook object for Sessions. Subclasses may be installed into a Session (or sessionmaker) using the extension keyword argument.
Execute after an instance is attached to a session.
This is called after an add, delete or merge.
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.
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.
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.
Execute after a commit has occured.
Note that this may not be per-flush if a longer running transaction is ongoing.
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.
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 occured, depending on whether or not the flush started its own transaction or participated in a larger transaction.
Execute after a rollback has occured.
Note that this may not be per-flush if a longer running transaction is ongoing.
Execute right before commit is called.
Note that this may not be per-flush if a longer running transaction is ongoing.
Execute before flush process has started.
instances is an optional list of objects which were passed to the flush() method.
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.
An event handler for individual attribute change events.
AttributeExtension is assembled within the descriptors associated with a mapped class.
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().
Receive a collection append event.
The returned value will be used as the actual value to be appended.
Receive a remove event.
No return value is defined.
Receive a set event.
The returned value will be used as the actual value to be set.
InstrumentationManager can be subclassed in order to receive class instrumentation events as well as to change how class instrumentation proceeds. This class exists for the purposes of integration with other object management frameworks which would like to entirely modify the instrumentation methodology of the ORM, and is not intended for regular usage. One possible exception is the InstrumentationManager.post_configure_attribute() method, which can be useful for adding extensions to all mapped attributes, though a much better way to do this will be available in a future release of SQLAlchemy.
For an example of InstrumentationManager, see the example Attribute Instrumentation.
User-defined class instrumentation extension.
The API for this class should be considered as semi-stable, and may change slightly with new releases.