How to Sum a column of a related table in Flask-SQLAlchemy

pay = db_session.query(func.sum(Orders.pay)).filter(Orders.shifts_id == shift_id).one()

pays = ( 
    s.query(Orders.shifts_id, sa.func.sum(Orders.pay))
    .filter(Orders.shifts_id.in_(list_of_shift_ids))
    .group_by(Orders.shifts_id)
    .all()
)