I've written a handful of primaryjoin and secondaryjoin attributes on
Relationships. This mechanism is flexible, but it's error-prone, and I
think that, at least for all the cases I've personally encountered, there
could be a better way. As an example, I have:
thing = Table('thing', metadata,
Column('thing_id', Integer, primary_key=True),
Column('favorite_group_id', Integer),
ForeignKeyConstraint(['thing_id', 'favorite_group_id'], ['rel.thing_id',
'rel.group_id'], use_alter=True, name='foobar'))
group = Table('group', metadata,
Column('group_id', Integer, primary_key=True))
rel = Table('rel', metadata,
Column('group_id', Integer, ForeignKey('group.group_id',
primary_key=True),
Column('thing_id', Integer, ForeignKey('thing.thing_id'),
primary_key=True))
IOW I have things and groups. The rel table is a many-to-many relation
between things and groups. A thing also may have a favorite group; if so,
there has to be a rel between that thing and its favorite group. I don't
have a foreign key directly from favorite_group_id because the equivalent
constraint is already implied by the existing foreign keys.
Everything is straightforward to map, except the favorite_group relation on
thing -- there's no foreign key. So I can set primary and primaryjoin, but
here's a different suggestion:
Add a couple of flags to ForeignKeyConstraint so that I can have three
kinds of ForeignKeyConstraint:
1. The normal kind: the mapper setup and the DDL both see it.
2. A mapper-only ForeignKeyConstraint: the mapper will look at it to deduce
join conditions, but there's no DDL.
3. A DDL-only ForeignKeyConstraint: There's DDL but the mapper won't see it.
So I could do:
thing = Table('thing', metadata,
Column('thing_id', Integer, primary_key=True),
Column('favorite_group_id', Integer),
ForeignKeyConstraint(['thing_id', 'favorite_group_id'], ['rel.thing_id',
'rel.group_id'], use_alter=True, use_for_mapper=False, name='foobar'),
ForeignKeyConstraint(['favorite_group_id'], ['group.group_id'],
use_alter=True, emit_ddl=False, name='foobar'))
And now the trivial mapper relations will all work with no fiddling.
Thoughts?
--Andy
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