From d1e9c25162eecb24dee50fcfe834a2735e53fb0e Mon Sep 17 00:00:00 2001
From: Simon Charette <charette.s@gmail.com>
Date: Fri, 8 Mar 2019 15:54:03 -0500
Subject: [PATCH] Refs #30188 -- Prevented double annotation of subquery when
 aggregated over.
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Thanks Can Sarıgöl for the suggested trimming approach.
---
 django/db/models/sql/query.py | 16 ++++++++++------
 tests/expressions/tests.py    | 22 +++++++++++++---------
 2 files changed, 23 insertions(+), 15 deletions(-)

diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 74aed551a3..d421efa7c3 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -383,12 +383,16 @@ class Query(BaseExpression):
                 new_expr, col_cnt = self.rewrite_cols(expr, col_cnt)
                 new_exprs.append(new_expr)
             elif isinstance(expr, (Col, Subquery)) or (expr.contains_aggregate and not expr.is_summary):
-                # Reference to column. Make sure the referenced column
-                # is selected.
-                col_cnt += 1
-                col_alias = '__col%d' % col_cnt
-                self.annotations[col_alias] = expr
-                self.append_annotation_mask([col_alias])
+                # Reference to column, subquery, or another aggregate. Make
+                # sure the expression is selected and reuse its alias if so.
+                for col_alias, selected_annotation in self.annotation_select.items():
+                    if selected_annotation == expr:
+                        break
+                else:
+                    col_cnt += 1
+                    col_alias = '__col%d' % col_cnt
+                    self.annotations[col_alias] = expr
+                    self.append_annotation_mask([col_alias])
                 new_exprs.append(Ref(col_alias, expr))
             else:
                 # Some other expression not referencing database values
diff --git a/tests/expressions/tests.py b/tests/expressions/tests.py
index bb448745c8..a51080ef13 100644
--- a/tests/expressions/tests.py
+++ b/tests/expressions/tests.py
@@ -553,16 +553,20 @@ class BasicExpressionsTests(TestCase):
 
     @skipUnlessDBFeature('supports_subqueries_in_group_by')
     def test_aggregate_subquery_annotation(self):
-        aggregate = Company.objects.annotate(
-            ceo_salary=Subquery(
-                Employee.objects.filter(
-                    id=OuterRef('ceo_id'),
-                ).values('salary')
-            ),
-        ).aggregate(
-            ceo_salary_gt_20=Count('pk', filter=Q(ceo_salary__gt=20)),
-        )
+        with self.assertNumQueries(1) as ctx:
+            aggregate = Company.objects.annotate(
+                ceo_salary=Subquery(
+                    Employee.objects.filter(
+                        id=OuterRef('ceo_id'),
+                    ).values('salary')
+                ),
+            ).aggregate(
+                ceo_salary_gt_20=Count('pk', filter=Q(ceo_salary__gt=20)),
+            )
         self.assertEqual(aggregate, {'ceo_salary_gt_20': 1})
+        # Aggregation over a subquery annotation doesn't annotate the subquery
+        # twice in the inner query.
+        self.assertLessEqual(ctx.captured_queries[0]['sql'].count('SELECT'), 4,)
 
     def test_explicit_output_field(self):
         class FuncA(Func):