[1.8.x] Fixed #24420 -- Allowed ordering by case expressions

Backport of ceaf31adff from master

django/db/models/expressions.py

@@ -630,6 +630,11 @@ class Ref(ExpressionNode):
     def set_source_expressions(self, exprs):
         self.source, = exprs
 
+    def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
+        # The sub-expression `source` has already been resolved, as this is
+        # just a reference to the name of `source`.
+        return self
+
     def relabeled_clone(self, relabels):
         return self
 

tests/annotations/tests.py

@@ -197,6 +197,24 @@ class NonAggregateAnnotationTestCase(TestCase):
         book = Book.objects.annotate(no_value=Value(None, output_field=IntegerField())).first()
         self.assertIsNone(book.no_value)
 
+    def test_order_by_annotation(self):
+        authors = Author.objects.annotate(other_age=F('age')).order_by('other_age')
+        self.assertQuerysetEqual(
+            authors, [
+                25, 29, 29, 34, 35, 37, 45, 46, 57,
+            ],
+            lambda a: a.other_age
+        )
+
+    def test_order_by_aggregate(self):
+        authors = Author.objects.values('age').annotate(age_count=Count('age')).order_by('age_count', 'age')
+        self.assertQuerysetEqual(
+            authors, [
+                (25, 1), (34, 1), (35, 1), (37, 1), (45, 1), (46, 1), (57, 1), (29, 2),
+            ],
+            lambda a: (a['age'], a['age_count'])
+        )
+
     def test_column_field_ordering(self):
         """
         Test that columns are aligned in the correct order for

tests/expressions_case/tests.py

@@ -995,6 +995,30 @@ class CaseExpressionTests(TestCase):
             transform=attrgetter('integer', 'integer2', 'test')
         )
 
+    def test_order_by_conditional_implicit(self):
+        self.assertQuerysetEqual(
+            CaseTestModel.objects.filter(integer__lte=2).annotate(test=Case(
+                When(integer=1, then=2),
+                When(integer=2, then=1),
+                default=3,
+                output_field=models.IntegerField(),
+            )).order_by('test', 'pk'),
+            [(2, 1), (2, 1), (1, 2)],
+            transform=attrgetter('integer', 'test')
+        )
+
+    def test_order_by_conditional_explicit(self):
+        self.assertQuerysetEqual(
+            CaseTestModel.objects.filter(integer__lte=2).annotate(test=Case(
+                When(integer=1, then=2),
+                When(integer=2, then=1),
+                default=3,
+                output_field=models.IntegerField(),
+            )).order_by(F('test').asc(), 'pk'),
+            [(2, 1), (2, 1), (1, 2)],
+            transform=attrgetter('integer', 'test')
+        )
+
 
 class CaseDocumentationExamples(TestCase):
     @classmethod