Simplified QuerySet field.null handling

QuerySet had previously some complex logic for dealing with nullable
fields in negated add_filter() calls. It seems the logic is leftover
from a time where the WhereNode wasn't as intelligent in handling
field__in=[] conditions.

Thanks to aaugustin for comments on the patch.

tests/regressiontests/queries/tests.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import
 
 import datetime
+from operator import attrgetter
 import pickle
 import sys
 
@@ -18,7 +19,7 @@ from .models import (Annotation, Article, Author, Celebrity, Child, Cover,
     ManagedModel, Member, NamedCategory, Note, Number, Plaything, PointerA,
     Ranking, Related, Report, ReservedName, Tag, TvChef, Valid, X, Food, Eaten,
     Node, ObjectA, ObjectB, ObjectC, CategoryItem, SimpleCategory,
-    SpecialCategory, OneToOneCategory)
+    SpecialCategory, OneToOneCategory, NullableName)
 
 
 class BaseQuerysetTest(TestCase):
@@ -1894,3 +1895,38 @@ class DefaultValuesInsertTest(TestCase):
             DumbCategory.objects.create()
         except TypeError:
             self.fail("Creation of an instance of a model with only the PK field shouldn't error out after bulk insert refactoring (#17056)")
+
+class NullInExcludeTest(TestCase):
+    def setUp(self):
+        NullableName.objects.create(name='i1')
+        NullableName.objects.create()
+
+    def test_null_in_exclude_qs(self):
+        none_val = '' if connection.features.interprets_empty_strings_as_nulls else None
+        self.assertQuerysetEqual(
+            NullableName.objects.exclude(name__in=[]),
+            ['i1', none_val], attrgetter('name'))
+        self.assertQuerysetEqual(
+            NullableName.objects.exclude(name__in=['i1']),
+            [none_val], attrgetter('name'))
+        self.assertQuerysetEqual(
+            NullableName.objects.exclude(name__in=['i3']),
+            ['i1', none_val], attrgetter('name'))
+        inner_qs = NullableName.objects.filter(name='i1').values_list('name')
+        self.assertQuerysetEqual(
+            NullableName.objects.exclude(name__in=inner_qs),
+            [none_val], attrgetter('name'))
+        # Check that the inner queryset wasn't executed - it should be turned
+        # into subquery above
+        self.assertIs(inner_qs._result_cache, None)
+
+    @unittest.expectedFailure
+    def test_col_not_in_list_containing_null(self):
+        """
+        The following case is not handled properly because
+        SQL's COL NOT IN (list containing null) handling is too weird to
+        abstract away.
+        """
+        self.assertQuerysetEqual(
+            NullableName.objects.exclude(name__in=[None]),
+            ['i1'], attrgetter('name'))