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

django/db/models/fields/related_descriptors.py

@@ -912,6 +912,33 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
                 False,
             )
 
+        @property
+        def constrained_target(self):
+            # If the through relation's target field's foreign integrity is
+            # enforced, the query can be performed solely against the through
+            # table as the INNER JOIN'ing against target table is unnecessary.
+            if not self.target_field.db_constraint:
+                return None
+            db = router.db_for_read(self.through, instance=self.instance)
+            if not connections[db].features.supports_foreign_keys:
+                return None
+            hints = {'instance': self.instance}
+            manager = self.through._base_manager.db_manager(db, hints=hints)
+            filters = {self.source_field_name: self.instance.pk}
+            # Nullable target rows must be excluded as well as they would have
+            # been filtered out from an INNER JOIN.
+            if self.target_field.null:
+                filters['%s__isnull' % self.target_field_name] = False
+            return manager.filter(**filters)
+
+        def exists(self):
+            constrained_target = self.constrained_target
+            return constrained_target.exists() if constrained_target else super().exists()
+
+        def count(self):
+            constrained_target = self.constrained_target
+            return constrained_target.count() if constrained_target else super().count()
+
         def add(self, *objs):
             if not rel.through._meta.auto_created:
                 opts = self.through._meta

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)

tests/many_to_many/tests.py

@@ -1,7 +1,13 @@
-from django.db import transaction
+from unittest import mock
-from django.test import TestCase
 
-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'])