SQLAlchemy - Writing a hybrid method for child count

Question

I\'m using Flask-SQLAlchemy, and I\'m trying to write a hybrid method in a parent model that returns the number of children it has, so I can use it for filtering, sorting, e

Answer 1

I think you can just use plain ol' len and hybrid_property to get the count:

@hybrid_property
def child_count(self):
    return len(self.children)

From the doc, that looks like it'd do the trick, unless I'm missing something?

duggars = db.session.query(Parent).filter(Parent.child_count > 17)

Answer 2

The code below shows it all.

class Parent(Base):
    __tablename__ = 'parents'
    # ...

    @hybrid_property
    def child_count(self):
        #return len(self.children)   # @note: use when non-dynamic relationship
        return self.children.count()# @note: use when dynamic relationship

    @child_count.expression
    def child_count(cls):
        return (select([func.count(Child.child_id)]).
                where(Child.parent_id == cls.parent_id).
                label("child_count")
                )

    @hybrid_method
    def child_count_ex(self, stime, etime):
        return len([_child for _child in self.children
            if stime <= _child.time <= etime ])

    @child_count_ex.expression
    def child_count_ex(cls, stime, etime):
        return (select([func.count(Child.child_id)]).
                where(Child.parent_id == cls.parent_id).
                where(Child.time >= stime).
                where(Child.time <= etime).
                label("child_count")
                )


# usage of expressions:
stime, etime = datetime.datetime(2012, 1, 1), datetime.datetime(2012, 1, 31)
qry = session.query(Parent)
#qry = qry.filter(Parent.child_count > 2)
qry = qry.filter(Parent.child_count_ex(stime, etime) > 0)