Semi-Join to a UNION construct.

[Christophe Biocca]

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.

[Mike Bayer]

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
)

"""






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[Christophe Biocca]

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.

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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper

http://www.sqlalchemy.org/

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[Mike Bayer]

On Mon, Feb 12, 2018 at 11:38 AM, Christophe Biocca
<christop...@gmail.com> wrote:
> 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.
>

you need to show me a mapping and SQL you want, then I can show you that.

>> > email to sqlalchemy+...@googlegroups.com.

>> > To post to this group, send email to sqlal...@googlegroups.com.
>> > Visit this group at https://groups.google.com/group/sqlalchemy.
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>>
>> --
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>> The Python SQL Toolkit and Object Relational Mapper
>>
>> http://www.sqlalchemy.org/
>>
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>
>
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>
> http://www.sqlalchemy.org/
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