def update_count(self):
        """ updates rating count and rating average """
        node_rating_count = self.node.rating_count
        node_rating_count.rating_count = self.node.rating_set.count()
        node_rating_count.rating_avg = self.node.rating_set.aggregate(rate=Avg('value'))['rate']

        # if all ratings are deleted the value will be None!
        if node_rating_count.rating_avg is None:
            # set to 0 otherwise we'll get an exception
            node_rating_count.rating_avg = 0

        node_rating_count.save()
updates rating count and rating average