Question

I don’t get it, but this code doesn’t call after_flush/before_flush/after_flush_postexec

# -*- coding: utf-8 -*-

from sqlalchemy.orm import scoped_session, sessionmaker
from sqlalchemy.orm.interfaces import SessionExtension

class AfterFlushExtension(SessionExtension):
    def before_commit(self, session):
        print "> before_commit"

    def after_commit(self, session):
        print "> after_commit"

    def before_flush(self, session, flush_context, instances):
        print '> before_flush'

    def after_flush(self, session, flush_context):
        print '> after_flush'

    def after_flush_postexec(self, session, flush_context):
        print '> after_flush_postexec'

session = scoped_session(sessionmaker(extension=AfterFlushExtension()))
session.flush()
session.commit()

And a result:

$ python ~/Dropbox/playground/python/sqlalchemy_hook_test/main.py 
> before_commit
> after_commit

Answer 1

Michael Bayer answered on SQLAlchemy’s mailing list https://groups.google.com/d/msg/sqlalchemy/GrMZGtJ-yc8/mCviGB6g9HYJ :

The flush events only fire off if
there’s actually something to be
flushed. It would be inefficient for
the events to be emitted for every
flush() as flush is in fact called a
great number of times, on every query,
assuming autoflush enabled. For this
reason a flush() with a session that
has no change events of any kind
quickly checks some flags and returns.

Think of before_flush() really being
called
before_flush_on_pending_changes() if
that helps.

I’ll check the docstrings to see if
any clarification is needed.

Thanks, Michael