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
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
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.
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
def number_of_children(self): if self.children: return len(self.children) return 0 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.