[2.2.x] Fixed #30325 -- Reverted "Fixed #29725 -- Removed unnecessary join in QuerySet.count() and exists() on a many-to-many relation."

This reverts commit 1299421cad due to
a regression with custom managers.

Backport of 5f7991c42c from master

django/db/models/fields/related_descriptors.py

@@ -929,33 +929,6 @@ 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, through_defaults=None):
             self._remove_prefetched_objects()
             db = router.db_for_write(self.through, instance=self.instance)

docs/releases/2.2.1.txt

@@ -29,3 +29,7 @@ Bugfixes
 * Prevented :djadmin:`makemigrations` from generating infinite migrations for
   check constraints and partial indexes when ``condition`` contains
   a :class:`~python:range` object (:ticket:`30350`).
+
+* Reverted an optimization in Django 2.2 (:ticket:`29725`) that caused the
+  inconsistent behavior of ``count()`` and ``exists()`` on a reverse
+  many-to-many relationship with a custom manager (:ticket:`30325`).

tests/many_to_many/models.py

@@ -55,13 +55,3 @@ 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

@@ -1,13 +1,9 @@
 from unittest import mock
 
-from django.db import connection, transaction
+from django.db import transaction
 from django.test import TestCase, skipUnlessDBFeature
-from django.test.utils import CaptureQueriesContext
 
-from .models import (
-    Article, InheritedArticleA, InheritedArticleB, NullablePublicationThrough,
-    NullableTargetArticle, Publication,
-)
+from .models import Article, InheritedArticleA, InheritedArticleB, Publication
 
 
 class ManyToManyTests(TestCase):
@@ -561,37 +557,3 @@ 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'])