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.

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.