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

            # 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()

        # 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>']

        # 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))