signals or triggers in SQLAlchemy

Question

does SQLAlchemy have something similar to Django\'s signal concept? Basically, I\'d like to trigger a few functions when I pre-save or post-save some entity objects. Thanks.

Answer 1

Here's my take at this problem, it uses Louie to dispatch signals:

dispatch.py

"""
Signals dispatching for SQLAlchemy mappers.
"""

import louie
from sqlalchemy.orm.interfaces import MapperExtension
import signals


class LouieDispatcherExtension(MapperExtension):
    """
    Dispatch signals using louie on insert, update and delete actions.
    """

    def after_insert(self, mapper, connection, instance):
        louie.send(signals.after_insert, instance.__class__,
                instance=instance)
        return super(LouieDispatcherExtension, self).after_insert(mapper,
                connection, instance)

    def after_delete(self, mapper, connection, instance):
        louie.send(signals.after_delete, instance.__class__,
                instance=instance)
        return super(LouieDispatcherExtension, self).after_delete(mapper,
                connection, instance)

    def after_update(self, mapper, connection, instance):
        louie.send(signals.after_update, instance.__class__,
                instance=instance)
        return super(LouieDispatcherExtension, self).after_update(mapper,
                connection, instance)

    def before_delete(self, mapper, connection, instance):
        louie.send(signals.before_delete, instance.__class__,
                instance=instance)
        return super(LouieDispatcherExtension, self).before_delete(mapper,
                connection, instance)

    def before_insert(self, mapper, connection, instance):
        louie.send(signals.before_insert, instance.__class__,
                instance=instance)
        return super(LouieDispatcherExtension, self).before_insert(mapper,
                connection, instance)

    def before_update(self, mapper, connection, instance):
        louie.send(signals.before_update, instance.__class__,
                instance=instance)
        return super(LouieDispatcherExtension, self).before_update(mapper,
                connection, instance)

signals.py

from louie import Signal


class after_delete(Signal): pass
class after_insert(Signal): pass
class after_update(Signal): pass
class before_delete(Signal): pass
class before_insert(Signal): pass
class before_update(Signal): pass

Sample usage:

class MyModel(DeclarativeBase):

    __mapper_args__ = {"extension": LouieDispatcherExtension()}

    ID = Column(Integer, primary_key=True)
    name = Column(String(255))

def on_insert(instance):
    print "inserted %s" % instance

louie.connect(on_insert, signals.after_insert, MyModel)