[1.5.x] Fixed #20091 -- Oracle null promotion for empty strings

Backpatch of e17fa9e

django/db/models/sql/query.py

@@ -1106,7 +1106,14 @@ def add_filter(self, filter_expr, connector=AND, negate=False, trim=False,
             # If value is a query expression, evaluate it
             value = SQLEvaluator(value, self, reuse=can_reuse)
             having_clause = value.contains_aggregate
-
+        # For Oracle '' is equivalent to null. The check needs to be done
+        # at this stage because join promotion can't be done at compiler
+        # stage. Using DEFAULT_DB_ALIAS isn't nice, but it is the best we
+        # can do here. Similar thing is done in is_nullable(), too.
+        if (connections[DEFAULT_DB_ALIAS].features.interprets_empty_strings_as_nulls and
+                lookup_type == 'exact' and value == ''):
+            value = True
+            lookup_type = 'isnull'
         for alias, aggregate in self.aggregates.items():
             if alias in (parts[0], LOOKUP_SEP.join(parts)):
                 entry = self.where_class()

tests/regressiontests/queries/tests.py

@@ -1533,7 +1533,6 @@ def test_tickets_8921_9188(self):
 
         # Nested queries are possible (although should be used with care, since
         # they have performance problems on backends like MySQL.
-
         self.assertQuerysetEqual(
             Annotation.objects.filter(notes__in=Note.objects.filter(note="n1")),
             ['<Annotation: a1>']
@@ -2142,3 +2141,11 @@ def test_ticket_17886(self):
         # so we can use INNER JOIN for it. However, we can NOT use INNER JOIN
         # for the b->c join, as a->b is nullable.
         self.assertEqual(str(qset.query).count('INNER JOIN'), 1)
+
+class EmptyStringPromotionTests(TestCase):
+    def test_empty_string_promotion(self):
+        qs = RelatedObject.objects.filter(single__name='')
+        if connection.features.interprets_empty_strings_as_nulls:
+            self.assertIn('LEFT OUTER JOIN', str(qs.query))
+        else:
+            self.assertNotIn('LEFT OUTER JOIN', str(qs.query))