Question :
Is there a way to make SQLAlchemy generate a query with a custom column that is a subquery that correlates with current row:
SELECT
tab1.id,
tab1.col1,
...,
(
SELECT count(1) FROM tab2
WHERE tab2.tab1_id = tab1.id
GROUP BY tab2.col1
) as cnt
FROM tab1
WHERE ...
LIMIT 100
using the ORM API?
session.query(Tab1, ?(subquery for additional column)?).filter(...).limit(100)
I’m using PostgreSQL 9.3 and old version of SQLAlchemy 0.9.8
Answer #1:
If you need this often, and/or the count is an integral part of your Tab1 model, you should use a hybrid property such as described in the other answer. If on the other hand you need this just for a single query, then you could just create the scalar subquery using Query.label(), or Query.as_scalar():
count_stmt = session.query(func.count(1)).
filter(Tab2.tab1_id == Tab1.id).
group_by(Tab2.col1).
label('cnt')
session.query(Tab1, count_stmt).filter(...).limit(100)
The subquery will automatically correlate what it can from the enclosing query.
Answer #2:
You can do this, but it works in a quite different way to how you have written it. You can create a property of Tab1 which depends on the relationship to tab2 (assuming that tab2.tab1_id is a foreign key, which it should be.
Your models look like this:
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
children = relationship("Child")
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id'))
as per the docs on relationships
Then you can add something like
@hybrid_property
def number_of_children(self):
if self.children:
return len(self.children)
return 0
@number_of_children.expression
def number_of_children(cls):
return (select([func.count(Child.id)])
.where(Child.cover_id == cls.id))
to the Parent model, as per this answer and more docs.
Once you’ve done this, you can filter on this property the same as any other column-based one.
