Generate sql with subquery as a column in select statement using SQLAlchemy

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, 
 .         


        

Answer 1

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.