You are mistaking about filtering an annotation.

   > Model.objects.annotate(Count('foos')).filter(foos__lt=10)

The only reason this doesn't work is because the property annotate creates isn't called `foos` by default. You just need to do this:

    Model.objects.annotate(foos=Count('foos')).filter(foos__lt=10)

and Django now knows to add a `HAVING COUNT(foos) < 10` clause.