On Aug 26, 2014, at 7:42 AM, alchemy1 <[email protected]> wrote:
> I have these classes, with a one-to-many relationship from User to Thing.
> Thing has a composite primary key of the user_uuid and the thing_type.
>
> class User(Base):
> ...
> uuid = Column(UUID, primary_key=True)
> things = relationship('Thing')
> ...
>
> class Thing(Base):
> ...
> user_uuid = Column(UUID, ForeignKey('user.uuid'), primary_key=True)
> thing_type = Column(CHAR(2), primary_key=True)
> ...
>
>
>
> I have a web page that has several forms, one for each Thing associated with
> the current user. A user can fill in say 3 of the forms and submit the form.
> Here's how I process the form:
>
> user = DBSession.query(User).options(joinedload('things')).filter(User.uuid
> == authenticated_userid(request)).one()
> new_thing_list = []
> for new_thing in new_things:
> new_thing_list.append(new_thing)
> user.things = new_thing_list # Overwrite the old list of Things
>
>
>
> I get the following error when trying to do it this way:
>
> AssertionError: Dependency rule tried to blank-out primary key column
> 'thing.user_uuid' on instance '<Thing at 0x7f80a45efc18>'
>
> I figure it's because overwriting the old list of Things without deleting
> each item first leads to orphans. So I change 'things' in User class to be:
>
> things = relationship('Thing', backref=backref('user', cascade='all,
> delete-orphan'))
>
> # Now I get this error
> # sqlalchemy.exc.ArgumentError: On Thing.user, delete-orphan cascade is not
> supported on a many-to-many or many-to-one relationship when single_parent is
> not set. Set single_parent=True on the relationship().
>
you put the "cascade='all, delete-oprhan'" on the *one-to-many* side, not the
many-to-one side. This is sometimes non-intuitive, so there's a green sidebar
that gets into this in this section:
http://docs.sqlalchemy.org/en/rel_0_9/orm/session.html#delete called
"ORM-level "delete" cascade vs. FOREIGN KEY level "ON DELETE" cascade".
This doc section should probably be linked to a little more in the relationship
docs.
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