PS - There is a "(" missing right after "lambda:"
On Feb 2, 5:26 pm, Manpreet Bhurji <[email protected]> wrote:
> A given A.id can only be in any one of field1id or field2id in an
> instance of B but a B is made up of different A's and hence field1id
> and field2id will be different A.id's which cannot appear in any other
> object of B.
>
> I'll try to illustrate the actual problem I had.
> We have 2 classes Person and Address.
> A person can have multiple addresses
>
> Person.py
> class Person(declarative_base):
> _primaryAddressId = Col()
> _primaryAddress = relationship(Address.Address, uselist=False,
> primaryjoin= lambda:Address.Address.id==Person.primaryAddressId),
> back_populate="_person")
> _secondaryAddressId = Col()
> _secondaryAddress = relationship(Address.Address, uselist=False,
> primaryjoin= lambda:Address.Address.id==Person.secondaryAddressId),
> back_populate="_person")
>
> Address.py
> class Address(declarative_base):
> _person=relationship("Person",
> primaryjoin="or_(Address._id==Person._primaryAddressId,
> Address.id==Person._secondaryAddressId)",uselist=False)
>
> In this case the following conditions apply -
> * A person has a primary and secondary address which no other person
> may have.
> * A primary address != a secondary address
> * Only 1 person may live at any address. (There are no families in
> this realm)
>
> If in the future I am to add a vacationAddress field or remove any
> previously present I will need to edit the primaryjoin in Address
> which is a drone. I am looking for a smarter way to accomplish this.
>
> Best,
> Aman
>
> On Feb 2, 4:36 pm, Michael Bayer <[email protected]> wrote:
>
>
>
>
>
>
>
> > why have "B.field1d" and separate "B.field2id" then if a given A.id can
> > only be in one and only one field1id/field2id ? If you had only one
> > column you could at least apply a UNIQUE constraint to it and enforce the
> > 1:1.
>
> > On Feb 2, 2012, at 4:31 PM, Manpreet Bhurji wrote:
>
> > > Thanks Michael. Really appreciate the quick reply.
>
> > > A syntax error in the primary was the reason that back_populate did
> > > not work. Fixing that solved it.
>
> > > I should have mentioned that field1 and field2 can only have a 1:1
> > > relationship towards A. I mean if field1 is assigned to an instance of
> > > A, it can't be assigned to any other. So any instance of A will have
> > > relationship "b" which only points to 1 B instance.
>
> > > The primary join in the A (when I am using back_populate) is now a
> > > long string which will need to be edited if I ever change the model B.
> > > Any ideas on how I might make this better?
>
> > > Regards,
> > > Aman
>
> > > On Feb 2, 3:36 pm, Michael Bayer <[email protected]> wrote:
> > >> 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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