Thanks,
Could you explain how to do contains_eager with an explicit query(). I tried
putting a query inside a call to contains_eager, but get an error:
ArgumentError: mapper option expects string key or list of attributes
Mark
On Aug 4, 2011, at 6:39 PM, Michael Bayer wrote:
> awkardly and inefficiently from a SQL perspective. contains_eager() with an
> explicit query() would produce better result
>
>
> from sqlalchemy import *
> from sqlalchemy.orm import *
> from sqlalchemy.ext.declarative import declarative_base
> Base = declarative_base()
> import datetime
>
> class A(Base):
> __tablename__ = 'a'
> id = Column(Integer, primary_key=True)
> bs = relationship("B")
>
> class B(Base):
> __tablename__ = 'b'
> id = Column(Integer, primary_key=True)
> a_id = Column(Integer, ForeignKey('a.id'))
> date = Column(Date)
>
> A.latest_b = relationship(B,
> primaryjoin=and_(
> A.id==B.a_id,
>
> B.date==select([func.max(B.date)]).where(B.a_id==A.id).correlate(A.__table__)
> )
> )
>
> e = create_engine('sqlite://', echo=True)
> Base.metadata.create_all(e)
> s = Session(e)
>
> s.add_all([
> A(bs=[
> B(date=datetime.date(2011, 10, 5)),
> B(date=datetime.date(2011, 8, 4)),
> B(date=datetime.date(2011, 9, 17)),
> ]),
> A(bs=[
> B(date=datetime.date(2011, 10, 5)),
> B(date=datetime.date(2011, 8, 4)),
> B(date=datetime.date(2011, 9, 17)),
> ]),
> ])
> s.commit()
>
> for obj in s.query(A).options(joinedload(A.latest_b)):
> print obj.latest_b
>
>
>
> On Aug 4, 2011, at 5:55 PM, Mark Erbaugh wrote:
>
>> Table A has a one to many relationship with Table B. There may be zero or
>> more rows in B for each row in A.
>>
>> I would like to have a query that retrieves all the rows in table A joined
>> with the "first" related row in table B (if one exists). In this case, each
>> row in table B has a DATE field and I want to retrieve the row with the
>> latest date. Is this possible using joinedload?
>>
>> Thanks,
>> Mark
>>
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>
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