Fixed #27279 -- Fixed a migration performance regression related to RenameModel operations.

Thanks Trac alias mtomiyoshi for the report, Marten Kenbeek for the initial
patch and Tim for the review.

django/db/migrations/operations/models.py

@@ -278,6 +278,11 @@ class RenameModel(ModelOperation):
         )
 
     def state_forwards(self, app_label, state):
+        # In cases where state doesn't have rendered apps, prevent subsequent
+        # reload_model() calls from rendering models for performance
+        # reasons. This method should be refactored to avoid relying on
+        # state.apps (#27310).
+        reset_apps = 'apps' not in state.__dict__
         apps = state.apps
         model = apps.get_model(app_label, self.old_name)
         model._meta.apps = apps
@@ -286,6 +291,8 @@ class RenameModel(ModelOperation):
             f for f in model._meta.get_fields(include_hidden=True)
             if f.auto_created and not f.concrete and (not f.hidden or f.many_to_many)
         )
+        if reset_apps:
+            del state.__dict__['apps']
         # Rename the model
         state.models[app_label, self.new_name_lower] = state.models[app_label, self.old_name_lower]
         state.models[app_label, self.new_name_lower].name = self.new_name

docs/releases/1.10.3.txt

@@ -11,3 +11,6 @@ Bugfixes
 
 * Allowed ``User.is_authenticated`` and ``User.is_anonymous`` properties to be
   tested for ``set`` membership (:ticket:`27309`).
+
+* Fixed a performance regression when running ``migrate`` in projects
+  with ``RenameModel`` operations (:ticket:`27279`).

tests/migrations/test_operations.py

@@ -5,7 +5,7 @@ import unittest
 from django.db import connection, migrations, models, transaction
 from django.db.migrations.migration import Migration
 from django.db.migrations.operations import CreateModel
-from django.db.migrations.state import ProjectState
+from django.db.migrations.state import ModelState, ProjectState
 from django.db.models.fields import NOT_PROVIDED
 from django.db.transaction import atomic
 from django.db.utils import IntegrityError
@@ -590,6 +590,26 @@ class OperationTests(OperationTestBase):
         self.assertEqual(definition[1], [])
         self.assertEqual(definition[2], {'old_name': "Pony", 'new_name': "Horse"})
 
+    def test_rename_model_state_forwards(self):
+        """
+        RenameModel operations shouldn't trigger the caching of rendered apps
+        on state without prior apps.
+        """
+        state = ProjectState()
+        state.add_model(ModelState('migrations', 'Foo', []))
+        operation = migrations.RenameModel('Foo', 'Bar')
+        operation.state_forwards('migrations', state)
+        self.assertNotIn('apps', state.__dict__)
+        self.assertNotIn(('migrations', 'foo'), state.models)
+        self.assertIn(('migrations', 'bar'), state.models)
+        # Now with apps cached.
+        apps = state.apps
+        operation = migrations.RenameModel('Bar', 'Foo')
+        operation.state_forwards('migrations', state)
+        self.assertIs(state.apps, apps)
+        self.assertNotIn(('migrations', 'bar'), state.models)
+        self.assertIn(('migrations', 'foo'), state.models)
+
     def test_rename_model_with_self_referential_fk(self):
         """
         Tests the RenameModel operation on model with self referential FK.