[1.10.x] 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.

Backport of 040bd7c9387cfbf70a543c507b3b9eeb8f2725dc from master

django/db/migrations/operations/models.py

@@ -273,6 +273,11 @@ 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
@@ -281,6 +286,8 @@ 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

@@ -4,7 +4,7 @@ import unittest
 
 from django.db import connection, migrations, models, transaction
 from django.db.migrations.migration import Migration
-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
@@ -579,6 +579,26 @@ 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.