django-sql-utils 0.1.1

Improved API for aggregating using Subquery

Django SQL Utils

This package provides utilities for working with Django querysets so that you can generate the SQL that you want, with an API you enjoy.

Subquery Aggregates

The Count aggregation in Django:

Parent.objects.annotate(child_count=Count('child'))

generates SQL like the following:

SELECT parent.*, Count(child.id) as child_count
FROM parent
JOIN child on child.parent_id = parent.id
GROUP BY parent.id

In many cases, this is not as performant as doing the count in a SUBQUERY instead of with a JOIN:

SELECT parent.*,
       (SELECT Count(id)
        FROM child
        WHERE parent_id = parent.id) as child_count
FROM parent

Django allows us to generate this SQL using The Subquery and OuterRef classes:

subquery = Subquery(Child.objects.filter(parent_id=OuterRef('id')).order_by()
                    .values('parent').annotate(count=Count('pk'))
                    .values('count'), output_field=IntegerField())
Parent.objects.annotate(child_count=Coalesce(subquery, 0))

Holy cow! It’s not trivial to figure what everything is doing in the above code and it’s not particularly good for maintenance. SubqueryAggregates allow you to forget all that complexity and generate the subquery count like this:

Parent.objects.annotate(child_count=SubqueryCount('child'))

Phew! Much easier to read and understand. It’s the same API as the original Count just specifying the Subquery version.

In addition to SubqueryCount, this package provides SubqueryMin and SubqueryMax. If you want to use other aggregates, you can use the generic SubqueryAggregate class:

from django.db.models import Avg, DecimalField

aggregate = SubqueryAggregate('child__age', aggregate=Avg,
                               output_field=DecimalField())
Parent.objects.annotate(avg_child_age=aggregate)

Or subclass SubqueryAggregate:

from django.db.models import Avg

class SubqueryAvg(SubqueryAggregate)
    aggregate = Avg
    unordered = True

Parent.objects.annotate(avg_child_age=SubqueryAvg('child__age')