[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 040bd7c938 from master

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.