Fixed #22073 - Ensure CreateTable operation handles backwards migration correctly when M2M fields are present

django/db/backends/schema.py

@@ -268,6 +268,11 @@ class BaseDatabaseSchemaEditor(object):
         """
         Deletes a model from the database.
         """
+        # Handle auto-created intermediary models
+        for field in model._meta.local_many_to_many:
+            if field.rel.through._meta.auto_created:
+                self.delete_model(field.rel.through)
+
         # Delete the table
         self.execute(self.sql_delete_table % {
             "table": self.quote_name(model._meta.db_table),

tests/migrations/test_operations.py

@@ -90,6 +90,46 @@ class OperationTests(MigrationTestBase):
         self.assertEqual(len(definition[2]), 0)
         self.assertEqual(definition[1][0], "Pony")
 
+    def test_create_model_m2m(self):
+        """
+        Test the creation of a model with a ManyToMany field and the
+        auto-created "through" model.
+        """
+        project_state = self.set_up_test_model("test_crmomm")
+        operation = migrations.CreateModel(
+            "Stable",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("ponies", models.ManyToManyField("Pony", related_name="stables"))
+            ]
+        )
+        # Test the state alteration
+        new_state = project_state.clone()
+        operation.state_forwards("test_crmomm", new_state)
+        # Test the database alteration
+        self.assertTableNotExists("test_crmomm_stable_ponies")
+        with connection.schema_editor() as editor:
+            operation.database_forwards("test_crmomm", editor, project_state, new_state)
+        self.assertTableExists("test_crmomm_stable")
+        self.assertTableExists("test_crmomm_stable_ponies")
+        self.assertColumnNotExists("test_crmomm", "ponies")
+        # Make sure the M2M field actually works
+        with atomic():
+            new_apps = new_state.render()
+            Pony = new_apps.get_model("test_crmomm", "Pony")
+            Stable = new_apps.get_model("test_crmomm", "Stable")
+            stable = Stable.objects.create()
+            p1 = Pony.objects.create(pink=False, weight=4.55)
+            p2 = Pony.objects.create(pink=True, weight=5.43)
+            stable.ponies.add(p1, p2)
+            self.assertEqual(stable.ponies.count(), 2)
+            stable.ponies.all().delete()
+        # And test reversal
+        with connection.schema_editor() as editor:
+            operation.database_backwards("test_crmomm", editor, new_state, project_state)
+        self.assertTableNotExists("test_crmomm_stable")
+        self.assertTableNotExists("test_crmomm_stable_ponies")
+
     def test_create_model_inheritance(self):
         """
         Tests the CreateModel operation on a multi-table inheritance setup.