OuterRef with possible NULL generates value = NULL instead of value IS NULL in COUNT() group by.

[Illia Petrov]

Let's imagine we have two models(db - PostgreSQL):

class Category(models.Model):
     name = models.CharField(max_length=255)

class Tag(models.Model):
    PRIORITY_CHOICES = (
       (1, "1"),
       (2, "2"),
       (3, "3"),
    )
    category = models.ForeignKey(Category, on_delete=models.SET_NULL, null=True, blank=True)
    priority = models.PositiveSmallIntegerField(default=PRIORITY_CHOICES[0][0], choices=PRIORITY_CHOICES)

Then we want to get all tags for category_id=3 or category_id=None with the number of tags with priority=3 grouped by categories(remember that we have null=True in the category field, so category_id could be empty):

from django.db.models import OuterRef, Subquery, Q, Count
import pprint

q = Q(category_id=3)|Q(category_id=None)
queryset = Tag.objects.filter(q, priority=3)
subquery = Tag.objects.filter(
     priority=OuterRef('priority'), 
     category_id=OuterRef('category_id')
).values('priority').annotate(priority_count=Count('*'))

result = queryset.annotate(
     priority_count=Subquery(subquery.values('priority_count'), output_field=IntegerField())
).values('id', 'category_id', 'priority', 'priority_count')
pprint.pprint(result)

Output result:

<QuerySet [{'id': 28, 'category_id': None, 'priority': 3, 'priority_count': None}, {'id': 39, 'category_id': 3, 'priority': 3, 'priority_count': 3}, {'id': 40, 'category_id': 3, 'priority': 3, 'priority_count': 3}, {'id': 41, 'category_id': 3, 'priority': 3, 'priority_count': 3}]>

Result SQL query:

SELECT "tag"."id",
       "tag"."category_id",
       "tag"."priority",
       (SELECT COUNT(*) AS "priority_count"
        FROM "tag" U0
        WHERE (U0."priority" = ("tag"."priority") AND
               U0."category_id" = ("tag"."category_id"))
        GROUP BY U0."priority") AS "priority_count"
FROM "tag"
WHERE (("tag"."category_id" = 3 OR "tag"."category_id" IS NULL) AND
        "tag"."priority" = 3)

Problem:
In output we can see priority_count == None for category_id == None.
The reason is clear, since in subquery we have U0."category_id" = ("tag"."category_id"))
So for now we ended up using UNION for separate queries with category_id=None and category_id=3.

Question:
Is it possible to do it without a union using Django ORM?
Will it be fixed in future?