On Nov 13, 2012, at 3:34 AM, tonthon wrote:
> Hi,
>
> I'm using polymorphism for some of my models and I'm wondering how I
> should use the with_polymorphic query method.
>
> Consider the following:
> """
> class A(Base):
> type_ = Column(Integer, nullable=False)
> arg = Column(String(20))
> __mapper_args__ = {'polymorphic_on': type_, 'polymorphic_identity': 0}
>
> class B(A):
> __mapper_args__ = {'polymorphic_identity': 1}
>
> class C(A):
> __mapper_args__ = {'polymorphic_identity': 2}
>
> class D(A):
> __mapper_args__ = {'polymorphic_identity': 3}
> """
>
> When I query :
>
> """
> session.query(A).with_polymorphic([B,C]).filter(A.arg == 'test')
> """
>
> I get D elements in my result.
> I've tried :
>
> """
> session.query(A).with_polymorphic([B,C]).filter(or_(B.arg=='test',
> C.arg=='test'))
> """
>
> But it doesn't work neither.
>
> Could somebody explain me what I did wrong and how I should do ?
with_polymorphic() is generally only useful with joined table inheritance (note
this is single table inheritance), and is used to add those subclass tables to
the Query so that you can filter() on their criterion, as well as to allow more
columns to be pulled in via a single query rather than needing to invoke
additional queries for subclass tables. It does not indicate a subset of
subclasses to be included.
If you want to load A's of type B, C, but not D, you'd need to do this manually
via the discriminator - since B, C and D are all As you normally will get back
all three as the rows determine.
session.query(A).filter(A.type_.in(0, 1, 2))
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