SQLAlchemy – condition on join fails with AttributeError: Neither ‘BinaryExpression’ object nor ‘Comparator’ object has an attribute ‘selectable’

I’m running SQLALchemy with Pyramid. I’m trying to run a query with a custom ‘join’ condition :

DBSession.query(A)\
        .outerjoin(A.b, B.a_id == A.id)\
        .all();

however the query fails the following error :

AttributeError: Neither ‘BinaryExpression’ object nor ‘Comparator’ object has an attribute ‘selectable’

The problem stems from the condition, as if I remove it, the query works :

DBSession.query(A)\
        .outerjoin(A.b)\
        .all();

I don’t understand the problem, as I follow the syntax described in the documentation :

q = session.query(User).join(Address, User.id==Address.user_id)

Does anyone see what’s going on ?

Best answer

Ok I saw it.
If you add a custom condition, the syntax is not .outerjoin(A.b, ...), but rather .outerjoin(B, ...)

They should accept both, really

(and the error message could be a little more explicit)