From 681f7e2b13846def9eff3d26ce1371d05fa90c4c Mon Sep 17 00:00:00 2001
From: Alex Aktsipetrov <aaktsipetrov@iponweb.net>
Date: Tue, 15 Oct 2019 01:59:43 +0300
Subject: Fixed #20577 -- Deferred filtering of prefetched related querysets.

Added internal interface to QuerySet that allows to defer next filter
call till .query is accessed. Used it to optimize prefetch_related().

Thanks Simon Charette for the review.
---
 tests/prefetch_related/tests.py | 31 +++++++++++++++++++++++++++++++
 tests/queryset_pickle/tests.py  |  6 ++++++
 2 files changed, 37 insertions(+)

(limited to 'tests')

diff --git a/tests/prefetch_related/tests.py b/tests/prefetch_related/tests.py
index 930ba9fbc8..5b944a456b 100644
--- a/tests/prefetch_related/tests.py
+++ b/tests/prefetch_related/tests.py
@@ -5,6 +5,7 @@ from django.core.exceptions import ObjectDoesNotExist
 from django.db import connection
 from django.db.models import Prefetch, QuerySet
 from django.db.models.query import get_prefetcher, prefetch_related_objects
+from django.db.models.sql import Query
 from django.test import TestCase, override_settings
 from django.test.utils import CaptureQueriesContext
 
@@ -291,6 +292,20 @@ class PrefetchRelatedTests(TestDataMixin, TestCase):
         sql = queries[-1]['sql']
         self.assertWhereContains(sql, self.author1.id)
 
+    def test_filter_deferred(self):
+        """
+        Related filtering of prefetched querysets is deferred until necessary.
+        """
+        add_q = Query.add_q
+        with mock.patch.object(
+            Query,
+            'add_q',
+            autospec=True,
+            side_effect=lambda self, q: add_q(self, q),
+        ) as add_q_mock:
+            list(Book.objects.prefetch_related('authors'))
+            self.assertEqual(add_q_mock.call_count, 1)
+
 
 class RawQuerySetTests(TestDataMixin, TestCase):
     def test_basic(self):
@@ -823,6 +838,22 @@ class CustomPrefetchTests(TestCase):
             with self.assertNumQueries(0):
                 self.assertEqual(person.cached_all_houses, all_houses)
 
+    def test_filter_deferred(self):
+        """
+        Related filtering of prefetched querysets is deferred until necessary.
+        """
+        add_q = Query.add_q
+        with mock.patch.object(
+            Query,
+            'add_q',
+            autospec=True,
+            side_effect=lambda self, q: add_q(self, q),
+        ) as add_q_mock:
+            list(House.objects.prefetch_related(
+                Prefetch('occupants', queryset=Person.objects.all())
+            ))
+            self.assertEqual(add_q_mock.call_count, 1)
+
 
 class DefaultManagerTests(TestCase):
 
diff --git a/tests/queryset_pickle/tests.py b/tests/queryset_pickle/tests.py
index 1b3f237819..9ef19cfa9d 100644
--- a/tests/queryset_pickle/tests.py
+++ b/tests/queryset_pickle/tests.py
@@ -212,6 +212,12 @@ class PickleabilityTestCase(TestCase):
         qs = Happening.objects.annotate(latest_time=models.Max('when'))
         self.assert_pickles(qs)
 
+    def test_filter_deferred(self):
+        qs = Happening.objects.all()
+        qs._defer_next_filter = True
+        qs = qs.filter(id=0)
+        self.assert_pickles(qs)
+
     def test_missing_django_version_unpickling(self):
         """
         #21430 -- Verifies a warning is raised for querysets that are
-- 
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