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)
  location = models.ForeignKey('Location', blank=True, to_field='uuid')

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

Location.objects.annotate(encounter_count=Count('subject__encounter'))