Django ORM按相关字段计数

Question

I have the following models:

class Profile(models.Model):
    pass

class Team(models.Model):
    ...
    members = models.ManyToManyField("main.Profile", through='main.TeamMember')

class TeamMember(models.Model):
    profile = models.ForeignKey('Profile')
    team = models.ForeignKey('Team')
    role = models.CharField(max_length=255)

So, I try to order teams by count of profiles with a certain role, and I had a query, which doesn't work:

Team.objects.filter(members__role='ExampleRole').annotate(example_role_count=Count('members')).order_by('example_role_count')

It raises the following Exception:

django.core.exceptions.FieldError: Related Field got invalid lookup: role

because role is a TeamMember field, not Profile. So, how can I achieve ordering by count of members with a specified role?

Answer 1

最后，找到了解决方案（仅在Django> = 1.11中有效）

queryset = Team.objects.annotate(example_role_count=Subquery(TeamMember.objects.filter(role='example_role', team=OuterRef('pk')).values('team').annotate(cnt=Count('pk')).values('cnt'), output_field=IntegerField())).order_by('-example_role_count')

Answer 2

If you want to filter the queryset by 'through' model field and then want to order the result set based on related field count,

from django.db.models import Count

# descending order
Team.objects.filter(teammember__role='ExampleRole').annotate(example_role_count=Count('members')).order_by('-example_role_count')