The easiest way to order by ratings is to add a GenericRelation to the Rating model from your model:

from django.contrib.contenttypes.fields import GenericRelation
from star_ratings.models import Rating

class Foo(models.Model):
    bar = models.CharField(max_length=100)
    ratings = GenericRelation(Rating, related_query_name='foos')

Foo.objects.filter(ratings__isnull=False).order_by('ratings__average')