[1.8.x] Fixed #24924 -- Join promotion for multiple Case expressions

Backport of 541f4ea546 from master

django/db/models/query_utils.py

@@ -87,10 +87,8 @@ class Q(tree.Node):
     def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
         # 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, split_subq=False)
-        joins_to_promote = [j for j in joins if j not in joins_before]
-        query.promote_joins(joins_to_promote)
+        query.promote_joins(joins)
         return clause
 
     @classmethod

docs/releases/1.8.3.txt

@@ -53,3 +53,7 @@ Bugfixes
 
 * Fixed queryset annotations when using ``Case`` expressions with ``exclude()``
   (:ticket:`24833`).
+
+* Corrected join promotion for multiple ``Case`` expressions. Annotating a
+  query with multiple  ``Case`` expressions could unexpectedly filter out
+  results (:ticket:`24924`).

tests/expressions_case/tests.py

@@ -1075,6 +1075,48 @@ class CaseExpressionTests(TestCase):
             lambda x: (x, x.foo)
         )
 
+    def test_join_promotion_multiple_annonations(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()
+                ),
+                bar=Case(
+                    When(fk_rel__pk=1, then=4),
+                    default=5,
+                    output_field=models.IntegerField()
+                ),
+            ),
+            [(o, 3, 5)],
+            lambda x: (x, x.foo, x.bar)
+        )
+        # 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()
+                ),
+                bar=Case(
+                    When(fk_rel__isnull=True, then=4),
+                    default=5,
+                    output_field=models.IntegerField()
+                ),
+            ),
+            [(o, 2, 4)],
+            lambda x: (x, x.foo, x.bar)
+        )
+
     def test_m2m_exclude(self):
         CaseTestModel.objects.create(integer=10, integer2=1, string='1')
         qs = CaseTestModel.objects.values_list('id', 'integer').annotate(