python - SQLAlchemy 0.6.5(和0.6.8)SessionExtension after_flush 不会被调用

Question

我不明白，但是这段代码没有调用 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()

结果:

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

Answer 1

Michael Bayer 在 SQLAlchemy 的邮件列表上回复 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.

谢谢，迈克尔