Count CompositePrimaryKey field targets toward function arity check

[Jacob Walls]

Calling a database function with an incorrect number of arguments raises TypeError if the function declares its ​arity. We could improve the DX around CompositePrimaryKey by leveraging this feature, instead of just ​advising to be careful.

The docs sort of imply some kind of error is raised ("# ERROR"), but this test passes:

tests/composite_pk/test_aggregate.py

@@ -1 +1 @@
 from django.db import NotSupportedError
-from django.db.models import Count, Q
+from django.db.models import Count, Max, Q
 from django.test import TestCase
 
 from .models import Comment, Tenant, User
@@ -32 +32 @@
         cls.comment_5 = Comment.objects.create(id=5, user=cls.user_3, text="barbaz")
         cls.comment_6 = Comment.objects.create(id=6, user=cls.user_3, text="baz")
 
+    def test_max_pk(self):
+        # Could raise TypeError instead.
+        self.assertEqual(Comment.objects.aggregate(Max("pk")), {'pk__max': 1})
+
     def test_users_annotated_with_comments_id_count(self):
         user_1, user_2, user_3 = User.objects.annotate(Count("comments__id")).order_by(
             "pk"

Relatedly, most of the built-in aggregate functions should set arity=1, which I'll include in a separate commit, but we can also discuss whether that needs to go through a deprecation path.

[Jacob Walls]

​PR