On 05/03/2010 10:33 PM, a...@vurve.com wrote:
> Hi All,
>
> This might be a noob question, but I wasn't able to to find the answer
> combing through the docs and google search. Given the following
> declarations
>
> Base = declarative_base()
>
> class A(Base):
> __tablename__ = 'A'
> id = Column(Integer, primary_key=True)
>
> class B(Base):
> __tablename__ = 'B'
> id = Column(Integer, primary_key=True)
> a_id = Column(Integer, ForeignKey('A.id'))
> a = relationship('A', backref=backref('b'))
>
> I want to query for all A where B is not null (essentially an inner
> join on A) with something like this
>
> session.query(A).options(joinedload('b')).filter(A.b != None)
>
> but it won't work because 'A.b' is a backref field. If I try
> filter('A.b' != None) it won't work either. So 2 part question:
>
> 1) is there a better way to do an inner join like this?
> 2) in general, how do you use backref fields inside of filter
> criteria?
>
>
The problem is not due to the backref: it is because A.b is a
one-to-many relationship, and "A.b != None" does not make sense for a
one-to-many relationship. If you really want inner-join semantics, then
you can do this (note that no filter is needed because that is
inherently part of the inner join):
session.query(A).join(A.b).options(contains_eager('b'))
Alternatively, if you want to ensure that the database returns only one
row per A, you can use a query like this, which will use an EXISTS clause:
session.query(A).filter(A.b.any())
Note that the query object will filter out duplicate As on the client
side in any case, so you will not notice a difference between these queries.
-Conor
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