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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