Django 1.11 Annotating a Subquery Aggregate

Question

This is a bleeding-edge feature that I\'m currently skewered upon and quickly bleeding out. I want to annotate a subquery-aggregate onto an existing queryset. Doing this bef

Answer 1

It's also possible to create a subclass of Subquery, that changes the SQL it outputs. For instance, you can use:

class SQCount(Subquery):
    template = "(SELECT count(*) FROM (%(subquery)s) _count)"
    output_field = models.IntegerField()

You then use this as you would the original Subquery class:

spaces = Space.objects.filter(carpark=OuterRef('pk')).values('pk')
Carpark.objects.annotate(space_count=SQCount(spaces))

You can use this trick (at least in postgres) with a range of aggregating functions: I often use it to build up an array of values, or sum them.