Fixed #31660 -- Fixed queryset crash when grouping by m2o relation.

Regression in 3a941230c8.

Thanks Tomasz Szymański for the report.

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.
         """
+        if hasattr(self.output_field, 'select_format'):
             return self.output_field.select_format(compiler, sql, params)
+        return sql, params
 
     @cached_property
     def identity(self):

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`).

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},
+        ])