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.

django/db/models/sql/query.py

@@ -1193,14 +1193,15 @@ class Query(object):
                             entry.negate()
                             self.where.add(entry, AND)
                             break
-                if not (lookup_type == 'in'
-                            and not hasattr(value, 'as_sql')
-                            and not hasattr(value, '_as_sql')
-                            and not value) and field.null:
-                    # Leaky abstraction artifact: We have to specifically
-                    # exclude the "foo__in=[]" case from this handling, because
-                    # it's short-circuited in the Where class.
-                    # We also need to handle the case where a subquery is provided
+                if field.null:
+                    # In SQL NULL = anyvalue returns unknown, and NOT unknown
+                    # is still unknown. However, in Python None = anyvalue is False
+                    # (and not False is True...), and we want to return this Python's
+                    # view of None handling. So we need to specifically exclude the
+                    # NULL values, and because we are inside NOT branch they will
+                    # be included in the final resultset. We are essentially creating
+                    # SQL like this here: NOT (col IS NOT NULL), where the first NOT
+                    # is added in upper layers of the code.
                     self.where.add((Constraint(alias, col, None), 'isnull', False), AND)
 
         if can_reuse is not None:

tests/regressiontests/queries/models.py

@@ -346,3 +346,9 @@ class OneToOneCategory(models.Model):
 
     def __unicode__(self):
         return "one2one " + self.new_name
+
+class NullableName(models.Model):
+    name = models.CharField(max_length=20, null=True)
+
+    class Meta:
+        ordering = ['id']

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'))