On May 30, 2009, at 6:57 AM, klaus wrote:
>
> Let's assume there is a foreign key A.fk -> B.id from table A to table
> B, and these tables and columns are mapped to corresponding classes.
> Then let's translate the foreign key to a reference A.ref -> B via a
> relation.
>
> We can easily join B onto A: session.query(A).join("ref") or
> session.query(A).join(A.ref).
>
> Joining in the opposite direction is much uglier:
> session.query(B).join
> ((A, A.fk == B.id)). It seems to need the basic foreign key column fk
> (if it is mapped). Is there a way to express this using only the
> relation? Something like session.query(B).join((A, A.ref == B)) or any
> notation like session.query(B).join((A, A.ref == B.self)) perhaps?
Usually you'd just have a backref so that you can spell out the join
in either direction using a relation name, which is the most direct
way of achieving this. I think this makes sense since you are
essentially configuring a "canned" join condition at the mapper level.
There is also a lesser used option which is
query(B).with_parent(<somea>) but that is not as flexible as a full
join. It used to be the case that you could use the same relation
in both directions - i.e. query(B).join((A, A.ref)) and it would often
"work", but in recent releases I put an explicit check against that
since it was preventing me from fixing some other bad behavior in
join().
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