How to sort by annotated Count() in a related model in Django

Question

I\'m building a food logging database in Django and I\'ve got a query related problem.

I\'ve set up my models to include (among other things) a Food model connected

Answer 1

Perhaps something like this?

Food.objects.filter(consumer__user=user)\
            .annotate(consumption_times=Count('consumer'))\
            .order_by('consumption_times')

Answer 2

I am having a very similar issue. Basically, I know that the SQL query you want is:

SELECT food.*, COUNT(IF(consumption.user_id=123,TRUE,NULL)) AS consumption_times
       FROM food LEFT JOIN consumption ON (food.id=consumption.food_id)
       ORDER BY consumption_times;

What I wish is that you could mix aggregate functions and F expression, annotate F expressions without an aggregate function, have a richer set of operations/functions for F expressions, and have virtual fields that are basically an automatic F expression annotation. So that you could do:

Food.objects.annotate(consumption_times=Count(If(F('consumer')==user,True,None)))\
            .order_by('consumtion_times')

Also, just being able more easily able to add your own complex aggregate functions would be nice, but in the meantime, here's a hack that adds an aggregate function to do this.

from django.db.models import aggregates,sql
class CountIf(sql.aggregates.Count):
    sql_template = '%(function)s(IF(%(field)s=%(equals)s,TRUE,NULL))'
sql.aggregates.CountIf = CountIf

consumption_times = aggregates.Count('consumer',equals=user.id)
consumption_times.name = 'CountIf'
rows = Food.objects.annotate(consumption_times=consumption_times)\
                   .order_by('consumption_times')