django count annotation field

Question

Django 1.3, Python 2.7

I have the following models (some irrelevant fields omitted):

class Encounter(models.Model):
  subject = models.ForeignKey('Subject', to_field='uuid')
  uuid = models.SlugField(max_length=36, unique=True, default=make_uuid, editable=False)

class Subject(models.Model):
  uuid = models.SlugField(max_length=36, unique=True, default=make_uuid, editable=False)

I calculated the number of encounters per subject using the following code:

e_in_s = Subject.objects.annotate(revcount = Count('encounter')).order_by('-revcount')

Now I would like to know how many subjects have exactly revcount encounters for each value of revcount. I tried the following:

sprime_in_e = e_in_s.values('revcount').annotate(countprime=Count('revcount'))

but got the error "Cannot compute Count('revcount'): 'revcount' is an aggregate". Any suggestions?

Note: I need the value produced by e_in_s as well as its annotation.

Answer 1

You can't use annotate with annotated values. However, you can use aggregate. See the docs.

Your queries will look like:

e_in_s = Subject.objects.annotate(revcount = Count('encounter')).order_by('-revcount')
sprime_in_e = e_in_s.values('revcount').aggregate(countprime=Count('revcount'))

If this doesn't suffice you'll need to use the extra feature or drop down to raw SQL.