Add a way to have an aggregate() result as a queryset

[Malcolm Tredinnick]

If one is trying to construct a complex queryset, involving nesting other querysets, there's currently no way to include the results of an aggregate() call. For example, consider

class Item(models.Model):
   name = models.CharField(max_length=10)

If I want to include the number of Items in some other queryset, such as

Foo.objects.filter(items__in=[...]).annotate(num=Count("items").filter(num=item_count)

We can't write item_count (which would be Items.objects.aggregate(Count("id"))) as an inner queryset here, since the aggregate() call returns a dictionary, not a queryset.

The return value of aggregate() is fine, but this ticket is about adding a version that does return a queryset. Right now, there's no way to fake this with annotate(...).values(...) since that always introduces a GROUP BY clause and the whole point is not to group by anything.

Not sure whether adding a parameter to aggregate() is the right idea, or adding something to the annotate() route to specify "no grouping whatsoever". This is all 1.2 timeframe stuff, but it's something to think about.

[Tobias McNulty]

Maybe I misunderstand the problem, but in (postgre)SQL there is no way to filter on an aggregate column, e.g.:

django_test=# SELECT "testapp_parent"."id", COUNT("testapp_child"."id") AS "num" 
FROM "testapp_parent" 
LEFT OUTER JOIN "testapp_child" ON ("testapp_parent"."id" = "testapp_child"."parent_id") 
WHERE "num" = 0 
GROUP BY "testapp_parent"."id";
ERROR:  column "num" does not exist
LINE 1: ...parent"."id" = "testapp_child"."parent_id") WHERE "num" = 0 ...

You can ORDER BY aggregate columns (so perhaps this idea is still useful) but I'm not sure it'll work with .filter(num=item_count)

[Tobias McNulty]

Hmm, my statements above are patently false (you can do that with HAVING). Forget I ever said anything.

[James Bennett]

1.2 is feature-frozen, moving this feature request off the milestone.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Simon Charette]

#28296 is a duplicate which happens to have more up-to-date discussion and and associated patch so I'll close this one as a duplicate.