>>> sub = session.query(
>>>     ChildA.parent_id,
>>>     func.count(ChildA.id).label('count'),
>>> )
>>> sub = sub.group_by(ChildA.parent_id)
>>> sub = query.subquery()
>>>
>>> # this works
>>> query1 = session.query(
>>>     ChildB.someattr,
>>>     sub.c.count,
>>> )
>>> query1 = query1.select_from(ChildB)
>>> print(query1.outerjoin((sub, sub.c.parent_id == ChildB.parent_id)))
SELECT child_b.someattr AS child_b_someattr, anon1.count AS anon1_count
FROM child_b LEFT OUTER JOIN (SELECT child_a.parent_id AS child_a_parent_id, COUNT(child_a.id) AS count
FROM child_a GROUP BY child_a.parent_id) AS anon1 ON anon1.parent_id = child_b.parent_id
>>>
>>> # this doesn't
>>> query2 = session.query(
>>>     sub.c.count,
>>>     ChildB.someattr,
>>> )
>>> query2 = query2.select_from(sub)
>>> print(query2.outerjoin((ChildB, sub.c.parent_id == ChildB.parent_id)))
Traceback (most recent call last):
    ...
InvalidRequestError: Could not find a FROM clause to join from
>>>
>>> # what i want:
>>> # SELECT anon1.count AS anon1_count, child_b.someattr AS child_b_someattr
>>> # FROM (SELECT child_a.parent_id AS child_a_parent_id, COUNT(child_a.id) AS count
>>> # FROM child_a GROUP BY child_a.parent_id) AS anon1 LEFT OUTER JOIN child_b ON anon1.parent_id = child_b.parent_id
>>>
>>> # this demonstrates select_from is ignored
>>> query3 = session.query(
>>>     ChildB.someattr,
>>>     sub.c.count,
>>> )
>>> query3 = query3.select_from(sub)
>>> print(query3.outerjoin((ChildB, sub.c.parent_id == ChildB.parent_id)))
SELECT child_b.someattr AS child_b_someattr, anon1.count AS anon1_count,
FROM (SELECT child_a.parent_id AS child_a_parent_id, COUNT(child_a.id) AS count
FROM child_a GROUP BY child_a.parent_id) AS anon1, child_b LEFT OUTER JOIN child_b ON anon1.parent_id = child_b.parent_id