From 937314dc05ee06eca91fcbb3ddf4380a60275b59 Mon Sep 17 00:00:00 2001
From: Mariusz Felisiak <felisiak.mariusz@gmail.com>
Date: Tue, 19 May 2020 12:30:49 +0200
Subject: [PATCH] [3.1.x] Fixed #31607 -- Fixed evaluated Subquery equality.

Regression in 691def10a0197d83d2d108bd9043b0916d0f09b4.

Backport of a125da6a7c79b1d4c55677d0bed6f9b1d7d77353 from master
---
 django/db/models/expressions.py |  9 ++++++++-
 tests/expressions/tests.py      | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index eb5fafc90b5..89332699d5f 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -1021,11 +1021,18 @@ class Subquery(Expression):
     def __init__(self, queryset, output_field=None, **extra):
         self.query = queryset.query
         self.extra = extra
+        # Prevent the QuerySet from being evaluated.
+        self.queryset = queryset._chain(_result_cache=[], prefetch_done=True)
         super().__init__(output_field)
 
     def __getstate__(self):
         state = super().__getstate__()
-        state.pop('_constructor_args', None)
+        args, kwargs = state['_constructor_args']
+        if args:
+            args = (self.queryset, *args[1:])
+        else:
+            kwargs['queryset'] = self.queryset
+        state['_constructor_args'] = args, kwargs
         return state
 
     def get_source_expressions(self):
diff --git a/tests/expressions/tests.py b/tests/expressions/tests.py
index b17a286d5df..72bca4f2053 100644
--- a/tests/expressions/tests.py
+++ b/tests/expressions/tests.py
@@ -511,6 +511,25 @@ class BasicExpressionsTests(TestCase):
             Employee.objects.exclude(company_point_of_contact_set=None).values('pk')
         )
 
+    def test_subquery_eq(self):
+        qs = Employee.objects.annotate(
+            is_ceo=Exists(Company.objects.filter(ceo=OuterRef('pk'))),
+            is_point_of_contact=Exists(
+                Company.objects.filter(point_of_contact=OuterRef('pk')),
+            ),
+            small_company=Exists(
+                queryset=Company.objects.filter(num_employees__lt=200),
+            ),
+        ).filter(is_ceo=True, is_point_of_contact=False, small_company=True)
+        self.assertNotEqual(
+            qs.query.annotations['is_ceo'],
+            qs.query.annotations['is_point_of_contact'],
+        )
+        self.assertNotEqual(
+            qs.query.annotations['is_ceo'],
+            qs.query.annotations['small_company'],
+        )
+
     def test_in_subquery(self):
         # This is a contrived test (and you really wouldn't write this query),
         # but it is a succinct way to test the __in=Subquery() construct.