[Sqlalchemy-tickets] Issue #2978: .listen() on Engine-level events fired from Connection does not work for in-progress Sessions if it's the first listener (zzzeek/sqlalchemy)

[Alan S. <iss...@bi...>]

New issue 2978: .listen() on Engine-level events fired from Connection does not work for in-progress Sessions if it's the first listener
https://bitbucket.org/zzzeek/sqlalchemy/issue/2978/listen-on-engine-level-events-fired-from

Alan Shreve:

Here's the succinct code explanation:

    sess = Session()
    sess.query(A).all()
    sqlalchemy.event.listen(engine, "before_execute", before_execute_cb)
    sess.query(A).all() # <--- USE THE SAME SESSION AS BEFORE

That second query *does not* trigger before_execute_cb. I expect it to trigger that. However, if you do this:

    # Register dummy listener to force Connection's dispatch descriptor to join with Engine's
    sqlalchemy.event.listen(engine, "before_execute", noop_fn)
    sess = Session()
    sess.query(A).all()
    sqlalchemy.event.listen(engine, "before_execute", before_execute_cb)
    sess.query(A).all() # <--- USE THE SAME SESSION AS BEFORE

This time, the second query *does* trigger before_execute_cb.

A full set of cases: 
https://gist.github.com/inconshreveable/9300571

This is a very subtle problem in the initialization of the dispatcher on Connection objects. The bug is here:

https://bitbucket.org/zzzeek/sqlalchemy/src/fa23570a338b53c813b6342fba0cacd4f5e61384/lib/sqlalchemy/engine/base.py?at=master#cl-66

I'm not well-versed enough in SQLAlchemy design to decide how to solve this, but my suggestion is probably:

Connection objects should *always* join with the Engine's dispatch object and when registering a new event listener on the Engine's dispatch object, it needs to iterate each open connection and set _has_events to True.