How to order by aggregate with conditions on fields of a relation

Question

My code:

class School(models.Model): pass

class Student(models.Model):
    school = models.ForeignKey(School)
    TYPE_CHOICES = (
        ('ug', 'Undergraduate'),
        ('gr', 'Graduate'),
        ('al', 'Alumnus'),
    )
    type = models.CharField(max_length=2)

How do I obtain a QuerySet of Schools ordered by the number of Undergraduate Students?

Answer 1

from django.db.models import Count
School.objects.filter(student__type='ug').annotate(
                      num_students=Count('student')
                     ).order_by('num_students')

See the documentation on the relationship between aggregate and filter clauses.