A tough relationship with Django

Question

Take a look at this model (it's hypothetical):

class Manufacturer(models.Model):
    #...

class Car(models.Model):
    manufacturer = models.ForeignKey(Manufacturer)
    #...

class City(models.Model):
    #...

class Manager(models.Model):
    manufacturer = models.ForeignKey(Manufacturer)
    city = models.ForeignKey(City)
    #...

What I want to query is: a list of cars and their manufacturer's managers (given a few conditions which are not important in the question), as well as their cities. This could somehow be done by the following code:

manager_car = defaultdict(list)
cars = Car.objects.select_related('manufacturer').filter(...)
for car in cars:
    managers = car.manufacturer.manager_set.select_related('city').filter(...)
    for manager in managers:
        #if <optional condition>:
        manager_car[manager].append(car)

Would list in a dictionary, the manufacturer's cars by manager, which is what I want. However, this code obviously runs as many queries as cars in the database.

How do I select all those instances at once?

Answer 1

Something like this, perhaps (just code off the top of my head):

cars = Car.objects.filter(...)
managers = {}
for manager in Manager.objects.filter(manufacturer__car__in=cars):
    manufacturers = managers.setdefault(manager.manufacturer_id, [])
    manufacturers.append(manager)
cars = list(cars)
for car in cars:
    car.managers = managers.get(car.manufacturer_id, [])