Functional fun!

functional.py

rt = Factory.route_trip()
drops = rt.droppoint_set.all()
managers = [drop.droppointmembership_set.filter(is_drop_manager=True)
    for drop in drops]
# flatten the list of managers:
managers = [manager for manager_list in managers for manager in manager_list]
credits = [manager.customer.credits.count() for manager in managers]
credits = reduce(lambda x, y: x + y, credits, 0)