From 0baea920c865ae2d40a4e1d9281e25327fc0dbe5 Mon Sep 17 00:00:00 2001
From: Sergey Fedoseev <fedoseev.sergey@gmail.com>
Date: Mon, 31 Jul 2017 01:01:45 +0500
Subject: [PATCH] Fixed #28453 -- Made __exact=None lookup use transforms.

---
 django/db/models/sql/query.py |  3 ++-
 tests/lookup/models.py        | 11 +++++++++++
 tests/lookup/tests.py         |  7 +++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 48bbd7c851..5e262d1765 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -984,7 +984,8 @@ class Query:
         if value is None:
             if lookups[-1] not in ('exact', 'iexact'):
                 raise ValueError("Cannot use None as a query value")
-            return True, ['isnull'], used_joins
+            lookups[-1] = 'isnull'
+            return True, lookups, used_joins
         elif hasattr(value, 'resolve_expression'):
             pre_joins = self.alias_refcount.copy()
             value = value.resolve_expression(self, reuse=can_reuse, allow_joins=allow_joins)
diff --git a/tests/lookup/models.py b/tests/lookup/models.py
index 2fa5b87755..1c7ea799a6 100644
--- a/tests/lookup/models.py
+++ b/tests/lookup/models.py
@@ -43,9 +43,20 @@ class Tag(models.Model):
         ordering = ('name', )
 
 
+class NulledTextField(models.TextField):
+    pass
+
+
+@NulledTextField.register_lookup
+class NulledTransform(models.Transform):
+    lookup_name = 'nulled'
+    template = 'NULL'
+
+
 class Season(models.Model):
     year = models.PositiveSmallIntegerField()
     gt = models.IntegerField(null=True, blank=True)
+    nulled_text_field = NulledTextField(null=True)
 
     def __str__(self):
         return str(self.year)
diff --git a/tests/lookup/tests.py b/tests/lookup/tests.py
index 41270cd19f..37e98f43e2 100644
--- a/tests/lookup/tests.py
+++ b/tests/lookup/tests.py
@@ -839,3 +839,10 @@ class LookupTests(TestCase):
              '<Article: Article 7>'],
             ordered=False
         )
+
+    def test_exact_none_transform(self):
+        """Transforms are used for __exact=None."""
+        Season.objects.create(year=1, nulled_text_field='not null')
+        self.assertFalse(Season.objects.filter(nulled_text_field__isnull=True))
+        self.assertTrue(Season.objects.filter(nulled_text_field__nulled__isnull=True))
+        self.assertTrue(Season.objects.filter(nulled_text_field__nulled__exact=None))