docs: def related_items_changed(self, instance, related_manager):
          """
          Calculates and saves the average rating.
          """
          ratings = [r.value for r in related_manager.all()]
          count = len(ratings)
          _sum = sum(ratings)
          average = _sum / count if count > 0 else 0
          setattr(instance, "%s_count" % self.related_field_name, count)
          setattr(instance, "%s_sum" % self.related_field_name, _sum)
          setattr(instance, "%s_average" % self.related_field_name, average)
          instance.save()