From 77278929c86168f075600d9d8c8e76a4792e672b Mon Sep 17 00:00:00 2001
From: Simon Charette <charette.s@gmail.com>
Date: Fri, 15 Dec 2023 21:00:59 -0500
Subject: [PATCH] Fixed #35042 -- Fixed a count() crash on combined queries.

Regression in 59bea9efd2768102fc9d3aedda469502c218e9b7.

Thanks Marcin for the report.
---
 django/db/models/sql/query.py |  6 +++---
 tests/aggregation/tests.py    | 15 +++++++++++++++
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 2bc16b1df351..7166b8cb5d96 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -520,11 +520,11 @@ def get_aggregation(self, using, aggregate_exprs):
                         self.model._meta.pk.get_col(inner_query.get_initial_alias()),
                     )
                 inner_query.default_cols = False
-                if not qualify:
+                if not qualify and not self.combinator:
                     # Mask existing annotations that are not referenced by
                     # aggregates to be pushed to the outer query unless
-                    # filtering against window functions is involved as it
-                    # requires complex realising.
+                    # filtering against window functions or if the query is
+                    # combined as both would require complex realiasing logic.
                     annotation_mask = set()
                     if isinstance(self.group_by, tuple):
                         for expr in self.group_by:
diff --git a/tests/aggregation/tests.py b/tests/aggregation/tests.py
index 62e9c6a27a5a..4408535228fd 100644
--- a/tests/aggregation/tests.py
+++ b/tests/aggregation/tests.py
@@ -2344,3 +2344,18 @@ def test_aggregate_reference_lookup_rhs_iter(self):
             max_book_author=Max("book__authors"),
         ).aggregate(count=Count("id", filter=Q(id__in=[F("max_book_author"), 0])))
         self.assertEqual(aggregates, {"count": 1})
+
+    def test_aggregate_combined_queries(self):
+        # Combined queries could have members in their values select mask while
+        # others have them in their annotation mask which makes annotation
+        # pruning complex to implement hence why it's not implemented.
+        qs = Author.objects.values(
+            "age",
+            other=Value(0),
+        ).union(
+            Book.objects.values(
+                age=Value(0),
+                other=Value(0),
+            )
+        )
+        self.assertEqual(qs.count(), 3)