Fixed #29725 -- Removed unnecessary join in QuerySet.count() and exists() on a many-to-many relation.

2 files changed, 53 insertions, 3 deletions

diff --git a/tests/many_to_many/models.py b/tests/many_to_many/models.py
index 22911654ab..7047a4e5d0 100644
--- a/tests/many_to_many/models.py
+++ b/tests/many_to_many/models.py
@@ -55,3 +55,13 @@ class InheritedArticleA(AbstractArticle):
 
 class InheritedArticleB(AbstractArticle):
     pass
+
+
+class NullableTargetArticle(models.Model):
+    headline = models.CharField(max_length=100)
+    publications = models.ManyToManyField(Publication, through='NullablePublicationThrough')
+
+
+class NullablePublicationThrough(models.Model):
+    article = models.ForeignKey(NullableTargetArticle, models.CASCADE)
+    publication = models.ForeignKey(Publication, models.CASCADE, null=True)
diff --git a/tests/many_to_many/tests.py b/tests/many_to_many/tests.py
index 5360b978c2..933eb23a7a 100644
--- a/tests/many_to_many/tests.py
+++ b/tests/many_to_many/tests.py
@@ -1,7 +1,13 @@
-from django.db import transaction
-from django.test import TestCase
+from unittest import mock
 
-from .models import Article, InheritedArticleA, InheritedArticleB, Publication
+from django.db import connection, transaction
+from django.test import TestCase, skipUnlessDBFeature
+from django.test.utils import CaptureQueriesContext
+
+from .models import (
+    Article, InheritedArticleA, InheritedArticleB, NullablePublicationThrough,
+    NullableTargetArticle, Publication,
+)
 
 
 class ManyToManyTests(TestCase):
@@ -554,3 +560,37 @@ class ManyToManyTests(TestCase):
             ]
         )
         self.assertQuerysetEqual(b.publications.all(), ['<Publication: Science Weekly>'])
+
+
+class ManyToManyQueryTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.article = Article.objects.create(headline='Django lets you build Web apps easily')
+        cls.nullable_target_article = NullableTargetArticle.objects.create(headline='The python is good')
+        NullablePublicationThrough.objects.create(article=cls.nullable_target_article, publication=None)
+
+    @skipUnlessDBFeature('supports_foreign_keys')
+    def test_count_join_optimization(self):
+        with CaptureQueriesContext(connection) as query:
+            self.article.publications.count()
+        self.assertNotIn('JOIN', query[0]['sql'])
+        self.assertEqual(self.nullable_target_article.publications.count(), 0)
+
+    def test_count_join_optimization_disabled(self):
+        with mock.patch.object(connection.features, 'supports_foreign_keys', False), \
+                CaptureQueriesContext(connection) as query:
+            self.article.publications.count()
+        self.assertIn('JOIN', query[0]['sql'])
+
+    @skipUnlessDBFeature('supports_foreign_keys')
+    def test_exists_join_optimization(self):
+        with CaptureQueriesContext(connection) as query:
+            self.article.publications.exists()
+        self.assertNotIn('JOIN', query[0]['sql'])
+        self.assertIs(self.nullable_target_article.publications.exists(), False)
+
+    def test_exists_join_optimization_disabled(self):
+        with mock.patch.object(connection.features, 'supports_foreign_keys', False), \
+                CaptureQueriesContext(connection) as query:
+            self.article.publications.exists()
+        self.assertIn('JOIN', query[0]['sql'])