[1.8.x] Fixed #24766 -- Added join promotion for Case expressions

Backport of be9d645346 from master

tests/expressions_case/tests.py

@@ -1031,6 +1031,38 @@ class CaseExpressionTests(TestCase):
             transform=attrgetter('integer', 'test')
         )
 
+    def test_join_promotion(self):
+        o = CaseTestModel.objects.create(integer=1, integer2=1, string='1')
+        # Testing that:
+        # 1. There isn't any object on the remote side of the fk_rel
+        #    relation. If the query used inner joins, then the join to fk_rel
+        #    would remove o from the results. So, in effect we are testing that
+        #    we are promoting the fk_rel join to a left outer join here.
+        # 2. The default value of 3 is generated for the case expression.
+        self.assertQuerysetEqual(
+            CaseTestModel.objects.filter(pk=o.pk).annotate(
+                foo=Case(
+                    When(fk_rel__pk=1, then=2),
+                    default=3,
+                    output_field=models.IntegerField()
+                ),
+            ),
+            [(o, 3)],
+            lambda x: (x, x.foo)
+        )
+        # Now 2 should be generated, as the fk_rel is null.
+        self.assertQuerysetEqual(
+            CaseTestModel.objects.filter(pk=o.pk).annotate(
+                foo=Case(
+                    When(fk_rel__isnull=True, then=2),
+                    default=3,
+                    output_field=models.IntegerField()
+                ),
+            ),
+            [(o, 2)],
+            lambda x: (x, x.foo)
+        )
+
 
 class CaseDocumentationExamples(TestCase):
     @classmethod