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

Backport of be9d645346 from master

django/db/models/query_utils.py

@@ -85,7 +85,12 @@ class Q(tree.Node):
         return clone
 
     def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
-        clause, _ = query._add_q(self, reuse, allow_joins=allow_joins)
+        # We must promote any new joins to left outer joins so that when Q is
+        # used as an expression, rows aren't filtered due to joins.
+        joins_before = query.tables[:]
+        clause, joins = query._add_q(self, reuse, allow_joins=allow_joins)
+        joins_to_promote = [j for j in joins if j not in joins_before]
+        query.promote_joins(joins_to_promote)
         return clause
 
     @classmethod

docs/releases/1.8.2.txt

@@ -13,3 +13,7 @@ Bugfixes
 
 * Fixed crash when reusing the same ``Case`` instance in a query
   (:ticket:`24752`).
+
+* Corrected join promotion for ``Case`` expressions. For example, annotating a
+  query with a  ``Case`` expression could unexpectedly filter out results
+  (:ticket:`24766`).

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