Refs #27985 -- Reallowed using __exact=None as an alias for __isnull=True if a custom lookup class with lookup_name != None is registered as the exact lookup.

Regression in 58da81a5a372a69f0bac801c412b57f3cce5f188 and prerequisite
for refs #28896.

1 files changed, 17 insertions, 0 deletions

diff --git a/tests/custom_lookups/tests.py b/tests/custom_lookups/tests.py
index bdb27a224a..9661aebc49 100644
--- a/tests/custom_lookups/tests.py
+++ b/tests/custom_lookups/tests.py
@@ -240,6 +240,23 @@ class LookupTests(TestCase):
             models.DateField._unregister_lookup(YearTransform)
             models.DateField._unregister_lookup(YearTransform, custom_transform_name)
 
+    def test_custom_exact_lookup_none_rhs(self):
+        """
+        __exact=None is transformed to __isnull=True if a custom lookup class
+        with lookup_name != 'exact' is registered as the `exact` lookup.
+        """
+        class CustomExactLookup(models.Lookup):
+            lookup_name = 'somecustomlookup'
+
+        field = Author._meta.get_field('birthdate')
+        OldExactLookup = field.get_lookup('exact')
+        author = Author.objects.create(name='author', birthdate=None)
+        try:
+            type(field).register_lookup(Exactly, 'exact')
+            self.assertEqual(Author.objects.get(birthdate__exact=None), author)
+        finally:
+            type(field).register_lookup(OldExactLookup, 'exact')
+
     def test_basic_lookup(self):
         a1 = Author.objects.create(name='a1', age=1)
         a2 = Author.objects.create(name='a2', age=2)