[1.7.x] Split ignores_nulls_in_unique_constraints feature.

Oracle and SQL Server don't have exactly the same limitations. It's
worth treating them differently.

Backport of cff59bed from master

django/db/backends/__init__.py

@@ -567,9 +567,14 @@ class BaseDatabaseFeatures(object):
     # Does the backend distinguish between '' and None?
     interprets_empty_strings_as_nulls = False
 
+    # Does the backend allow inserting duplicate NULL rows in a nullable
+    # unique field? All core backends implement this correctly, but other
+    # databases such as SQL Server do not.
+    supports_nullable_unique_constraints = True
+
     # Does the backend allow inserting duplicate rows when a unique_together
-    # constraint exists, but one of the unique_together columns is NULL?
-    ignores_nulls_in_unique_constraints = True
+    # constraint exists and some fields are nullable but not all of them?
+    supports_partially_nullable_unique_constraints = True
 
     can_use_chunked_reads = True
     can_return_id_from_insert = False

django/db/backends/oracle/base.py

@@ -107,7 +107,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_zoneinfo_database = pytz is not None
     supports_bitwise_or = False
     can_defer_constraint_checks = True
-    ignores_nulls_in_unique_constraints = False
+    supports_partially_nullable_unique_constraints = False
     has_bulk_insert = True
     supports_tablespaces = True
     supports_sequence_reset = False

tests/distinct_on_fields/tests.py

@@ -8,6 +8,7 @@ from .models import Tag, Celebrity, Fan, Staff, StaffTag
 
 
 @skipUnlessDBFeature('can_distinct_on_fields')
+@skipUnlessDBFeature('supports_nullable_unique_constraints')
 class DistinctOnTests(TestCase):
     def setUp(self):
         t1 = Tag.objects.create(name='t1')

tests/model_forms/tests.py

@@ -660,7 +660,7 @@ class UniqueTest(TestCase):
         form = TripleForm({'left': '1', 'middle': '3', 'right': '1'})
         self.assertTrue(form.is_valid())
 
-    @skipUnlessDBFeature('ignores_nulls_in_unique_constraints')
+    @skipUnlessDBFeature('supports_nullable_unique_constraints')
     def test_unique_null(self):
         title = 'I May Be Wrong But I Doubt It'
         form = BookForm({'title': title, 'author': self.writer.pk})

tests/model_formsets/tests.py

@@ -661,7 +661,7 @@ class ModelFormsetTest(TestCase):
         self.assertEqual(book1.title, 'Flowers of Evil')
         self.assertEqual(book1.notes, 'English translation of Les Fleurs du Mal')
 
-    @skipUnlessDBFeature('ignores_nulls_in_unique_constraints')
+    @skipUnlessDBFeature('supports_partially_nullable_unique_constraints')
     def test_inline_formsets_with_nullable_unique_together(self):
         # Test inline formsets where the inline-edited object has a
         # unique_together constraint with a nullable member

tests/one_to_one_regress/tests.py

@@ -229,7 +229,7 @@ class OneToOneRegressionTests(TestCase):
 
         # Several instances of the origin are only possible if database allows
         # inserting multiple NULL rows for a unique constraint
-        if connection.features.ignores_nulls_in_unique_constraints:
+        if connection.features.supports_nullable_unique_constraints:
             UndergroundBar.objects.create()
 
             # When there are several instances of the origin