Django QuerySet ordering by number of reverse ForeignKey matches

Question

I have the following Django models:

class Foo(models.Model):
    title = models.CharField(_(u'Title'), max_length=600)

class Bar(models.Model):
    foo = models.ForeignKey(Foo)
    eg_id = models.PositiveIntegerField(_(u'Example ID'), default=0)

I wish to return a list of Foo objects which have a reverse relationship with Bar objects that have a eg_id value contained in a list of values. So I have:

id_list = [7, 8, 9, 10]
qs = Foo.objects.filter(bar__eg_id__in=id_list)

How do I order the matching Foo objects according to the number of related Bar objects which have an eg_id value in the id_list?

Answer 1

Use Django's lovely aggregation features.

from django.db.models import Count
qs = Foo.objects.filter(
           bar__eg_id__in=id_list
      ).annotate(
           bar_count=Count('bar')
      ).order_by('bar_count')

Answer 2

You can do that by using aggregation, and more especifically annotation and order_by.

In your example, it would be:

from django.db.models import Count

id_list = [7, 8, 9, 10]
qs = Foo.objects.filter(bar__eg_id__in=id_list)
qs = qs.annotate(count=Count("bar"))
qs = qs.order_by('-count')