Django: Order a model by a many-to-many field

Question

I am writing a Django application that has a model for People, and I have hit a snag. I am assigning Role objects to people using a Many-To-Many relationship - where Roles have

Answer 1

Django 1.1 (currently beta) adds aggregation support. Your query can be done with something like:

from django.db.models import Max
People.objects.annotate(max_weight=Max('roles__weight')).order_by('-max_weight')

This sorts people by their heaviest roles, without returning duplicates.

The generated query is:

SELECT people.id, people.name, MAX(role.weight) AS max_weight
FROM people LEFT OUTER JOIN people_roles ON (people.id = people_roles.people_id)
            LEFT OUTER JOIN role ON (people_roles.role_id = role.id)
GROUP BY people.id, people.name
ORDER BY max_weight DESC