On Feb 2, 2012, at 3:16 PM, Manpreet Bhurji wrote:
> I have a structure like this
> A.py
> class A:
> _some_fields
>
> B.py
> class B:
> _field1id = Column(...)
> _field1 = relationship( A.A, primaryjoin=lambda:A.A.id==B.field1id,
> backref="b", uselist=False )
> _field2id = Column(...)
> _field2 = relationship( A.A, primaryjoin=lambda:A.A.id==B.field2id,
> backref="b", uselist=False )
>
> I know this will not work because the relationship field1 has already
> created "b" on A.
> I have tried using back_populates="b" in place of backref in class B
> and adding
> _b = relationship( "B", primaryjoin="or_(A.id==B.field1id,
> A.id==B.field2id)" ) to A
>
> Any ideas on how I would go about this?
What object would A.b return if its "id" were present both in the "field1id" of
one particular "B", and in the "field2id" of another ? Would A.b be a
collection of both ? If so this requires a union of some kind, most easily
done in Python. Naming the backrefs "b_from_field1" and "b_from_field2", we'd
say:
class A(Base):
@property
def b(self):
return self.b_from_field1.union(self.b_from_field2)
it's also possible to create a relationship A.b that selects from both field1id
and field2id by creating a UNION and then mapping B to that union using
non_primary=True, then using that mapper as the target of A.b, though this is a
little more involved.
In both cases A.b is read-only since it cant be determined if new B() entries
would be established via the B._field1 relationship or the B._field2
relationship.
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