annotate() and aggregate() for generically-related data. also a handy function for filtering GFK-model querysets.
Use django's GenericRelation where possible, as this can make the queries generated more efficient by using a JOIN rather than a subquery.
# install from pypi pip install django-generic-aggregation # or install via git pip install -e git+git://github.com/coleifer/django-generic-aggregation.git#egg=generic_aggregation
The examples below assume the following simple models:
class Rating(models.Model): rating = models.IntegerField() object_id = models.IntegerField() content_type = models.ForeignKey(ContentType) content_object = GenericForeignKey(ct_field='content_type', fk_field='object_id') class Food(models.Model): name = models.CharField(max_length=50) ratings = generic.GenericRelation(Rating) # reverse generic relation
You want to figure out which items are highest rated (generic_annotate)
from django.db.models import Avg food_qs = Food.objects.filter(name__startswith='a') generic_annotate(food_qs, Rating, Avg('ratings__rating')) # you can mix and match queryset / model generic_annotate(food_qs, Rating.objects.all(), Avg('ratings__rating'))
You want the average rating for all foods that start with 'a' (generic_aggregate)
food_qs = Food.objects.filter(name__startswith='a') generic_aggregate(food_qs, Rating, Avg('ratings__rating'))
You want to only display ratings for foods that start with 'a' (generic_filter)
food_qs = Food.objects.filter(name__startswith='a') generic_filter(Rating.objects.all(), food_qs)