Refs #24397 -- Sped up model reloading in ProjectState.

Created bulk_update() context manager on StateApps. Sped up unregistering
models in reload_models() by using this context mananger.

django/db/migrations/state.py

@@ -2,6 +2,7 @@ from __future__ import unicode_literals
 
 import copy
 from collections import OrderedDict
+from contextlib import contextmanager
 
 from django.apps import AppConfig
 from django.apps.registry import Apps, apps as global_apps
@@ -111,11 +112,9 @@ class ProjectState(object):
             related_models.add((app_label, model_name))
 
             # Unregister all related models
+            with self.apps.bulk_update():
                 for rel_app_label, rel_model_name in related_models:
                     self.apps.unregister_model(rel_app_label, rel_model_name)
-            # Need to do it once all models are unregistered to avoid corrupting
-            # existing models' _meta
-            self.apps.clear_cache()
 
             states_to_be_rendered = []
             # Gather all models states of those models that will be rerendered.
@@ -226,6 +225,18 @@ class StateApps(Apps):
             labels = (".".join(model_key) for model_key in self._pending_operations)
             raise ValueError(msg % ", ".join(labels))
 
+    @contextmanager
+    def bulk_update(self):
+        # Avoid clearing each model's cache for each change. Instead, clear
+        # all caches when we're finished updating the model instances.
+        ready = self.ready
+        self.ready = False
+        try:
+            yield
+        finally:
+            self.ready = ready
+            self.clear_cache()
+
     def render_multiple(self, model_states):
         # We keep trying to render the models in a loop, ignoring invalid
         # base errors, until the size of the unrendered models doesn't
@@ -234,7 +245,7 @@ class StateApps(Apps):
         if not model_states:
             return
         # Prevent that all model caches are expired for each render.
-        self.ready = False
+        with self.bulk_update():
             unrendered_models = model_states
             while unrendered_models:
                 new_unrendered_models = []
@@ -244,7 +255,6 @@ class StateApps(Apps):
                     except InvalidBasesError:
                         new_unrendered_models.append(model)
                 if len(new_unrendered_models) == len(unrendered_models):
-                self.ready = True
                     raise InvalidBasesError(
                         "Cannot resolve bases for %r\nThis can happen if you are inheriting models from an "
                         "app with migrations (e.g. contrib.auth)\n in an app with no migrations; see "
@@ -252,8 +262,6 @@ class StateApps(Apps):
                         "for more" % (new_unrendered_models, get_docs_version())
                     )
                 unrendered_models = new_unrendered_models
-        self.ready = True
-        self.clear_cache()
 
     def clone(self):
         """

tests/migrations/test_state.py

@@ -160,6 +160,22 @@ class StateTests(TestCase):
         self.assertEqual([mgr.args for name, mgr in food_order_manager_state.managers],
                          [('a', 'b', 1, 2), ('x', 'y', 3, 4)])
 
+    def test_apps_bulk_update(self):
+        """
+        StateApps.bulk_update() should update apps.ready to False and reset
+        the value afterwards.
+        """
+        project_state = ProjectState()
+        apps = project_state.apps
+        with apps.bulk_update():
+            self.assertFalse(apps.ready)
+        self.assertTrue(apps.ready)
+        with self.assertRaises(ValueError):
+            with apps.bulk_update():
+                self.assertFalse(apps.ready)
+                raise ValueError()
+        self.assertTrue(apps.ready)
+
     def test_render(self):
         """
         Tests rendering a ProjectState into an Apps.