def METHOD_NAME(self, obj):
    ct = ContentType.objects.get_for_model(obj)
    try:
        rating = OverallRating.objects.filter(object_id=obj.pk, content_type=ct).aggregate(r=Avg("rating"))["r"]
        return float(str(rating or "0"))
    except OverallRating.DoesNotExist:
        return 0.0