Right, I can unpack the relationship between A and B into column
expressions manually, but this will break for `secondaryjoin` relationships.
I guess I can unpack those manually as well, by introducing the secondary
table expression when needed. I will try that.
On 12 February 2018 at 11:24, Mike Bayer <mike...@zzzcomputing.com> wrote:
> On Sat, Feb 10, 2018 at 6:17 PM, Christophe Biocca
> <christophe.bio...@gmail.com> wrote:
> > Let's say I have 2 models, A and B, with relationships A.bs
> (haslist=True)
> > and B.a (haslist=False)
> >
> > Doing a semi join is trivial:
> >
> > session.query(A).filter(A.bs.any(SomeConditionOnB))
> > session.query(B).filter(B.a.has(SomeConditionOnA))
> >
> > Doing a regular join is also trivial:
> >
> > session.query(A).join(A.bs).filter(SomeConditionOnB)
> > session.query(B).filter(B.a).filter(SomeConditionOnA)
> >
> > I'd like to generate the following statement:
> >
> > SELECT * FROM a WHERE (EXISTS
> > (
> > SELECT 1 FROM b WHERE SomeConditionOnB AND b.a_id = a.id
> > UNION SELECT 1 FROM b WHERE SomeOtherConditionOnB AND b.a_id =
> a.id
> > )
> > )
> >
> > Because in my case `WHERE EXISTS( foo UNION bar )` is much, much faster
> than
> > `WHERE EXISTS( foo ) OR EXISTS( bar )`.
> >
> > I've figured out how do do it as a regular join (with `aliased(B,
> > b1.union(b2))`) but that won't work (it'll return some rows repeatedly
> due
> > to JOIN's behaviour vs the semijoin of EXISTS).
> >
> > `has` and `any` both take a criterion object (?) which means giving it an
> > aliased object won't work either.
> >
> > I've tried directly using `A.b.expression` and ClauseAdapter based on my
> > perusing of the source code, but it won't work with tables with secondary
> > joins and the like.
>
> there's no joins in the statement you describe, you can get that
> close-to-identical SQL using union() and exists() normally, full
> example:
>
> from sqlalchemy import *
> from sqlalchemy.orm import *
> from sqlalchemy.ext.declarative import declarative_base
> from sqlalchemy.ext.declarative import declared_attr
>
> Base = declarative_base()
>
>
> 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(ForeignKey('a.id'))
>
> session = Session()
>
>
> """
> I'd like to generate the following statement:
>
> SELECT * FROM a WHERE (EXISTS
> (
> SELECT 1 FROM b WHERE SomeConditionOnB AND b.a_id = a.id
> UNION SELECT 1 FROM b WHERE SomeOtherConditionOnB AND b.a_id =
> a.id
> )
> )
> """
>
> q = session.query(A).filter(
> session.query(B.id).filter(B.id > 5).filter(B.a_id ==
> A.id).correlate(A).
> union(
> session.query(B.id).filter(B.id < 10).filter(B.a_id == A.id).
> correlate(A)
> ).exists()
> )
>
> print(q)
>
>
> """
> SELECT a.id AS a_id FROM a WHERE EXISTS
> (
> SELECT 1 FROM (
> SELECT b.id AS b_id FROM b WHERE b.id > :id_1 AND b.a_id =
> a.id
> UNION SELECT b.id AS b_id FROM b WHERE b.id < :id_2 AND
> b.a_id = a.id
> ) AS anon_1
> )
>
> """
>
>
>
>
>
>
> >
> > --
> > SQLAlchemy -
> > The Python SQL Toolkit and Object Relational Mapper
> >
> > http://www.sqlalchemy.org/
> >
> > To post example code, please provide an MCVE: Minimal, Complete, and
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> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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