Fixed #25320 -- Reverted ManyToManyField.null to False for backwards compatibility.

Thanks Tom Christie for the report and review.

django/contrib/admin/filters.py

@@ -176,8 +176,16 @@ class RelatedFieldListFilter(FieldListFilter):
         self.title = self.lookup_title
         self.empty_value_display = model_admin.get_empty_value_display()
 
+    @property
+    def include_empty_choice(self):
+        """
+        Return True if a "(None)" choice should be included, which filters
+        out everything except empty relationships.
+        """
+        return self.field.null or (self.field.is_relation and self.field.many_to_many)
+
     def has_output(self):
-        if self.field.null:
+        if self.include_empty_choice:
             extra = 1
         else:
             extra = 0
@@ -204,7 +212,7 @@ class RelatedFieldListFilter(FieldListFilter):
                 }, [self.lookup_kwarg_isnull]),
                 'display': val,
             }
-        if self.field.null:
+        if self.include_empty_choice:
             yield {
                 'selected': bool(self.lookup_val_isnull),
                 'query_string': cl.get_query_string({

django/db/models/fields/related.py

@@ -2304,8 +2304,6 @@ class ManyToManyField(RelatedField):
 
         self.db_table = db_table
         self.swappable = swappable
-        # Many-to-many fields are always nullable.
-        self.null = True
 
     def check(self, **kwargs):
         errors = super(ManyToManyField, self).check(**kwargs)
@@ -2552,7 +2550,6 @@ class ManyToManyField(RelatedField):
     def deconstruct(self):
         name, path, args, kwargs = super(ManyToManyField, self).deconstruct()
         # Handle the simpler arguments.
-        del kwargs["null"]
         if self.db_table is not None:
             kwargs['db_table'] = self.db_table
         if self.remote_field.db_constraint is not True:

tests/admin_filters/tests.py

@@ -523,6 +523,16 @@ class ListFiltersTests(TestCase):
         self.assertEqual(choice['selected'], True)
         self.assertEqual(choice['query_string'], '?books_contributed__id__exact=%d' % self.django_book.pk)
 
+        # With one book, the list filter should appear because there is also a
+        # (None) option.
+        Book.objects.exclude(pk=self.djangonaut_book.pk).delete()
+        filterspec = changelist.get_filters(request)[0]
+        self.assertEqual(len(filterspec), 2)
+        # With no books remaining, no list filters should appear.
+        Book.objects.all().delete()
+        filterspec = changelist.get_filters(request)[0]
+        self.assertEqual(len(filterspec), 0)
+
     def test_relatedonlyfieldlistfilter_foreignkey(self):
         modeladmin = BookAdminRelatedOnlyFilter(Book, site)
 

tests/model_fields/test_field_flags.py

@@ -216,3 +216,9 @@ class FieldFlagsTests(test.SimpleTestCase):
                 reverse_field = field.remote_field
                 self.assertEqual(field.model, reverse_field.related_model)
                 self.assertEqual(field.related_model, reverse_field.model)
+
+    def test_null(self):
+        # null isn't well defined for a ManyToManyField, but changing it to
+        # True causes backwards compatibility problems (#25320).
+        self.assertFalse(AllFieldsModel._meta.get_field('m2m').null)
+        self.assertTrue(AllFieldsModel._meta.get_field('reverse2').null)