[3.1.x] Fixed #32200 -- Fixed grouping by ExpressionWrapper() with Q objects.

Thanks Gordon Wrigley for the report.

Regression in df32fd42b8.

Backport of fe9c7ded29 from master

django/db/models/expressions.py

@@ -866,9 +866,13 @@ class ExpressionWrapper(Expression):
         return [self.expression]
 
     def get_group_by_cols(self, alias=None):
-        expression = self.expression.copy()
-        expression.output_field = self.output_field
-        return expression.get_group_by_cols(alias=alias)
+        if isinstance(self.expression, Expression):
+            expression = self.expression.copy()
+            expression.output_field = self.output_field
+            return expression.get_group_by_cols(alias=alias)
+        # For non-expressions e.g. an SQL WHERE clause, the entire
+        # `expression` must be included in the GROUP BY clause.
+        return super().get_group_by_cols()
 
     def as_sql(self, compiler, connection):
         return compiler.compile(self.expression)

docs/releases/3.1.4.txt

@@ -21,3 +21,6 @@ Bugfixes
 * Fixed a regression in Django 3.1 that caused a crash of auto-reloader for
   certain invocations of ``runserver`` on Windows with Python 3.7 and below
   (:ticket:`32202`).
+
+* Fixed a regression in Django 3.1 that caused the incorrect grouping by a
+  ``Q`` object annotation (:ticket:`32200`).

tests/annotations/tests.py

@@ -199,6 +199,18 @@ class NonAggregateAnnotationTestCase(TestCase):
         self.assertEqual(book.isnull_pubdate, False)
         self.assertEqual(book.rating_count, 1)
 
+    @skipUnlessDBFeature('supports_boolean_expr_in_select_clause')
+    def test_grouping_by_q_expression_annotation(self):
+        authors = Author.objects.annotate(
+            under_40=ExpressionWrapper(Q(age__lt=40), output_field=BooleanField()),
+        ).values('under_40').annotate(
+            count_id=Count('id'),
+        ).values('under_40', 'count_id')
+        self.assertCountEqual(authors, [
+            {'under_40': False, 'count_id': 3},
+            {'under_40': True, 'count_id': 6},
+        ])
+
     def test_aggregate_over_annotation(self):
         agg = Author.objects.annotate(other_age=F('age')).aggregate(otherage_sum=Sum('other_age'))
         other_agg = Author.objects.aggregate(age_sum=Sum('age'))