Michael Bayer wrote:
[snip]
> Whats missing here is the context.
I want to define ORM relations in the mapper that I can access without
having to do a manual join.
Normally you'd do this:
mapper(A, a_table,
properties={
'bs': relation(B , backref='a',
primaryjoin=and_(a_table.c_id == b_table.c.a_id,
b_table.c.status == 'FOO'),
})
But I'd like to automate this:
mapper(A, a_table,
properties={
'bs': my_own_relation(B , backref='a'),
})
my_own_relation behaves like relation, except it adds an extra clause
restricting the query, say, b_table.c.status == 'FOO'. It should have
access to the parent and child tables so it can do this generically.
I've tried to accomplish this by overriding _determine_joins and
manipulating self.primaryjoin after the default is set up, but I think
that breaks because backrefs have already been set up earlier. I've
hacked around that now I think, but it's not very pretty.
> From what you describe, "use the
> primaryjoin and amend with an extra filter", that is just:
>
> query(Parent).join(Parent.children).filter(Child.foo==bar)
This presents a potential alternative implementation strategy where this
is implemented using properties on the model classes that do something
like this.
Regards,
Martijn
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