Accounted for a possible race condition in model state rendering.

mutant/compat.py

@@ -45,11 +45,14 @@ def clear_opts_related_cache(model_class):
         Clear the specified model and its children opts related cache.
         """
         opts = model_class._meta
-        children = [
-            related_object.model
-            for related_object in opts.get_all_related_objects()
-            if related_object.field.rel.parent_link
-        ]
+        if hasattr(opts, '_related_objects_cache'):
+            children = [
+                related_object.model
+                for related_object in opts.get_all_related_objects()
+                if related_object.field.rel.parent_link
+            ]
+        else:
+            children = []
         for attr in _opts_related_cache_attrs:
             try:
                 delattr(opts, attr)
@@ -77,6 +80,11 @@ def bulk_update(self):
                 self.ready = ready
                 self.clear_cache()
 
+        def get_model(self, app_label, model_name=None):
+            if model_name is None:
+                app_label, model_name = app_label.split('.')
+            return self.all_models[app_label][model_name]
+
         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

mutant/db/models.py

@@ -40,7 +40,6 @@ def get_model_state(cls):
 
     @classmethod
     def render_state(cls):
-        apps = StateApps([], {})
         state = cls.get_model_state()
         model_states = {(state.app_label, state.name): state}
         for _name, field in state.fields:
@@ -56,6 +55,7 @@ def render_state(cls):
                             cls._meta.apps.get_model(base), exclude_rels=True
                         )
                         model_states[base_model_state.app_label, base_model_state.name] = base_model_state
+        apps = StateApps([], {})
         apps.render_multiple(model_states.values())
         return apps.all_models[state.app_label][state.name.lower()]
 

mutant/models/model/__init__.py

@@ -280,7 +280,12 @@ def construct(self, force_create=False, existing_model_class=None):
             remove_from_app_cache(existing_model_class)
             existing_model_class.mark_as_obsolete()
 
-        model_class = state.render(apps)
+        try:
+            model_class = state.render(apps)
+        except RuntimeError:
+            # Account for race conditions between the removal from the apps
+            # and the rendering of the new state.
+            return apps.get_model(state.app_label, state.name)
         model_class._checksum = checksum
         for attr, value in attrs.items():
             setattr(model_class, attr, value)