Fixed #24397 -- Sped up rendering multiple model states.

Set apps.ready to False when rendering multiple models. This prevents
that the cache on Model._meta is expired on all models after each time a
single model is rendered. Prevented that Apps.clear_cache() refills the
cache on Apps.get_models(), so that the wrong value cannot be cached
when cloning a StateApps.

django/apps/registry.py

@@ -326,8 +326,11 @@ class Apps(object):
         # the relation tree and the fields cache.
         self.get_models.cache_clear()
         if self.ready:
-            for model in self.get_models(include_auto_created=True):
+            # Circumvent self.get_models() to prevent that the cache is refilled.
-                model._meta._expire_cache()
+            # This particularly prevents that an empty value is cached while cloning.
+            for app_config in self.app_configs.values():
+                for model in app_config.get_models(include_auto_created=True):
+                    model._meta._expire_cache()
 
 
 apps = Apps(installed_apps=None)

django/db/migrations/state.py

@@ -236,6 +236,10 @@ class StateApps(Apps):
         # base errors, until the size of the unrendered models doesn't
         # decrease by at least one, meaning there's a base dependency loop/
         # missing base.
+        if not model_states:
+            return
+        # Prevent that all model caches are expired for each render.
+        self.ready = False
         unrendered_models = model_states
         while unrendered_models:
             new_unrendered_models = []
@@ -245,6 +249,7 @@ 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 "
@@ -252,6 +257,8 @@ 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):
         """