[1.8.x] Fixed #24835 -- Fixed QuerySet.exists() after an annotation with Count()

QuerySet.exists() incorrectly handled query.group_by = True
case (grouping by all select fields), causing GROUP BY
expressions to be wiped along with select fields.

Backport of 801a84ae32 from master

django/db/models/sql/query.py

@@ -472,6 +472,9 @@ class Query(object):
     def has_results(self, using):
         q = self.clone()
         if not q.distinct:
+            if q.group_by is True:
+                q.add_fields((f.attname for f in self.model._meta.concrete_fields), False)
+                q.set_group_by()
             q.clear_select_clause()
         q.clear_ordering(True)
         q.set_limits(high=1)

docs/releases/1.8.3.txt

@@ -14,3 +14,6 @@ Bugfixes
 
 * Fixed crash during :djadmin:`makemigrations` if a migrations module either
   is missing ``__init__.py`` or is a file (:ticket:`24848`).
+
+* Fixed ``exists()`` returning incorrect results after annotation with
+  ``Count()`` (:ticket:`24835`).

tests/annotations/tests.py

@@ -245,6 +245,10 @@ class NonAggregateAnnotationTestCase(TestCase):
             lambda a: (a['age'], a['age_count'])
         )
 
+    def test_annotate_exists(self):
+        authors = Author.objects.annotate(c=Count('id')).filter(c__gt=1)
+        self.assertFalse(authors.exists())
+
     def test_column_field_ordering(self):
         """
         Test that columns are aligned in the correct order for