From 78ad4b4b0201003792bfdbf1a7781cbc9ee03539 Mon Sep 17 00:00:00 2001
From: Mariusz Felisiak <felisiak.mariusz@gmail.com>
Date: Mon, 8 Jun 2020 07:21:54 +0200
Subject: [PATCH] Fixed #31660 -- Fixed queryset crash when grouping by m2o
 relation.
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Regression in 3a941230c85b2702a5e1cd97e17251ce21057efa.

Thanks Tomasz Szymański for the report.
---
 django/db/models/expressions.py |  4 +++-
 docs/releases/3.0.8.txt         |  3 +++
 tests/annotations/tests.py      | 27 ++++++++++++++++++++++++---
 3 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index 6bd14716926..76f8fded41f 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -383,7 +383,9 @@ class BaseExpression:
         Custom format for select clauses. For example, EXISTS expressions need
         to be wrapped in CASE WHEN on Oracle.
         """
-        return self.output_field.select_format(compiler, sql, params)
+        if hasattr(self.output_field, 'select_format'):
+            return self.output_field.select_format(compiler, sql, params)
+        return sql, params
 
     @cached_property
     def identity(self):
diff --git a/docs/releases/3.0.8.txt b/docs/releases/3.0.8.txt
index e355f0a0ffa..d21eac37c8e 100644
--- a/docs/releases/3.0.8.txt
+++ b/docs/releases/3.0.8.txt
@@ -11,3 +11,6 @@ Bugfixes
 
 * Fixed messages of ``InvalidCacheKey`` exceptions and ``CacheKeyWarning``
   warnings raised by cache key validation (:ticket:`31654`).
+
+* Fixed a regression in Django 3.0.7 that caused a queryset crash when grouping
+  by a many-to-one relationship (:ticket:`31660`).
diff --git a/tests/annotations/tests.py b/tests/annotations/tests.py
index 142c23ead6c..c1ac0516acd 100644
--- a/tests/annotations/tests.py
+++ b/tests/annotations/tests.py
@@ -1,11 +1,13 @@
 import datetime
 from decimal import Decimal
+from unittest import skipIf
 
 from django.core.exceptions import FieldDoesNotExist, FieldError
+from django.db import connection
 from django.db.models import (
-    BooleanField, CharField, Count, DateTimeField, Exists, ExpressionWrapper,
-    F, Func, IntegerField, Max, NullBooleanField, OuterRef, Q, Subquery, Sum,
-    Value,
+    BooleanField, Case, CharField, Count, DateTimeField, Exists,
+    ExpressionWrapper, F, Func, IntegerField, Max, NullBooleanField, OuterRef,
+    Q, Subquery, Sum, Value, When,
 )
 from django.db.models.expressions import RawSQL
 from django.db.models.functions import Length, Lower
@@ -632,3 +634,22 @@ class NonAggregateAnnotationTestCase(TestCase):
             datetime.date(2008, 6, 23),
             datetime.date(2008, 11, 3),
         ])
+
+    @skipIf(
+        connection.vendor == 'mysql' and 'ONLY_FULL_GROUP_BY' in connection.sql_mode,
+        'GROUP BY optimization does not work properly when ONLY_FULL_GROUP_BY '
+        'mode is enabled on MySQL, see #31331.',
+    )
+    def test_annotation_aggregate_with_m2o(self):
+        qs = Author.objects.filter(age__lt=30).annotate(
+            max_pages=Case(
+                When(book_contact_set__isnull=True, then=Value(0)),
+                default=Max(F('book__pages')),
+                output_field=IntegerField(),
+            ),
+        ).values('name', 'max_pages')
+        self.assertCountEqual(qs, [
+            {'name': 'James Bennett', 'max_pages': 300},
+            {'name': 'Paul Bissex', 'max_pages': 0},
+            {'name': 'Wesley J. Chun', 'max_pages': 0},
+        ])