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
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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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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