From 0e7a45fca09623e7bad9408a838e522b3cca8d65 Mon Sep 17 00:00:00 2001
From: Hannes Ljungberg <hannes.ljungberg@gmail.com>
Date: Mon, 19 Oct 2020 22:36:24 +0200
Subject: [PATCH] Fixed #32126 -- Fixed grouping by Case() annotation without
 cases.

Co-authored-by: Simon Charette <charettes@users.noreply.github.com>
---
 django/db/models/expressions.py |  5 +++++
 tests/expressions_case/tests.py | 25 +++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index 43b8920d951..13918467ee3 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -1068,6 +1068,11 @@ class Case(Expression):
             sql = connection.ops.unification_cast_sql(self.output_field) % sql
         return sql, sql_params
 
+    def get_group_by_cols(self, alias=None):
+        if not self.cases:
+            return self.default.get_group_by_cols(alias)
+        return super().get_group_by_cols(alias)
+
 
 class Subquery(Expression):
     """
diff --git a/tests/expressions_case/tests.py b/tests/expressions_case/tests.py
index 2a101eeee16..54c53ef4def 100644
--- a/tests/expressions_case/tests.py
+++ b/tests/expressions_case/tests.py
@@ -1148,6 +1148,31 @@ class CaseExpressionTests(TestCase):
             lambda x: x[1:]
         )
 
+    def test_aggregation_empty_cases(self):
+        tests = [
+            # Empty cases and default.
+            (Case(output_field=IntegerField()), None),
+            # Empty cases and a constant default.
+            (Case(default=Value('empty')), 'empty'),
+            # Empty cases and column in the default.
+            (Case(default=F('url')), ''),
+        ]
+        for case, value in tests:
+            with self.subTest(case=case):
+                self.assertQuerysetEqual(
+                    CaseTestModel.objects.values('string').annotate(
+                        case=case,
+                        integer_sum=Sum('integer'),
+                    ).order_by('string'),
+                    [
+                        ('1', value, 1),
+                        ('2', value, 4),
+                        ('3', value, 9),
+                        ('4', value, 4),
+                    ],
+                    transform=itemgetter('string', 'case', 'integer_sum'),
+                )
+
 
 class CaseDocumentationExamples(TestCase):
     @classmethod