Question

I need a query that can return the records from table A that have greater than COUNT records in table B. The query needs to be able to go in line with other filters that might be applied on table A.

Example case study:

I have a person and appointments table. I am looking for all people who have been in for 5 or more appointments. It must also support extra filter statements on the person table, such as age > 18.

EDIT — SOLUTION

subquery = db.session.query(Appointment.id_person, 
                            func.count('*').label('person_count')) \
                     .group_by(Appointment.id_person).subquery()
qry = db.session.query(Person) \
                .outerjoin((subquery, Person.id == subquery.c.id_person)) \
                .order_by(Person.id).filter(subquery.c.person_count >= 5).filter(Person.dob <= '1992-10-29')

Answer 1

Use a subquery:

SELECT * from person
WHERE PersonID IN 
  (SELECT PersonId FROM appointments
   GROUP BY PersonId
   HAVING COUNT(*) >= 5)
AND dob > 25