Ahhh, ok! I was missing two important bits of info:
1) I didn't know about the backref() function. I was just using
backref='users' in my relationship() args.
2) Thus, I didn't realize that using cascade_backrefs=False as an arg
to relationship() would apply the setting in the wrong direction.
Using "backref=backref('users', cascade_backrefs=False)" has fixed my
problem. Thanks for the quick response!
On Jun 23, 4:34 pm, Michael Bayer <mike...@zzzcomputing.com> wrote:
> On Jun 23, 2011, at 7:12 PM, Robert Rollins wrote:
>
>
>
> > I'm using SQLAlchemy 0.7.1 on a MySQL 5.1.57 database, and I'm getting
> > unexpected behavior with the cascade_backrefs=False parameter for my
> > many-to-many relationships.
>
> > groups = relationship(ZKGroup, secondary=user_groups,
> > backref='users', cascade_backrefs=False)
>
> > def __init__(self, username, groups)
> > self.username = username
> > self.groups = groups
>
> > Now, I would expect the cascade_backrefs=False option to prevent newly
> > created ZKUser objects with a persistent ZKGroup object in their
> > 'groups' list to not be added to the session. But the ZKUser object
> > appears to being getting added to the session anyway. I can tell
> > because this code:
>
> > Is this a bug with cascade_backrefs=False? Or is it just not meant to
> > be used with many-to-many relationships, or something like that?
>
> I was pretty sure this came down to which side you put the flag on, keeping
> in mind that *I* didn't know which side it should be on. Apparently its
> the side that would be receiving cascades via a backref, not the one that
> would be sending them:
>
> groups = relationship(ZKGroup,
> secondary=user_groups,
> backref=backref('users', cascade_backrefs=False))
>
> That is, the event is initiated by "self.groups", which then hits group.users
> on the backref, but then the new User is not cascaded due to the flag.
>
> The docstring for relationship() is accurate here, from the perspective that
> the "cascade_backrefs" goes on the side that would be receiving the event,
> not the one initiating it, but it only speaks in terms of
> one-to-many/many-to-one, so even with all that typing I did it's still not
> clear. But with backref events, it doesn't really matter what type of
> pattern is on each side.
>
> The documentation for the flag in the "Session / Cascades" section, well
> looking there it looks like the mappings are nonsensical, geez....fixing that
> now.
>
>
>
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