Fix infinite recursion in swagger.py for circular models, fixes #190

Author: Drarok

flask_restplus/swagger.py

@@ -550,6 +550,8 @@ def register_model(self, model):
         if name not in self.api.models:
             raise ValueError('Model {0} not registered'.format(name))
         specs = self.api.models[name]
+        if name in self._registered_models:
+            return ref(model)
         self._registered_models[name] = specs
         if isinstance(specs, ModelBase):
             for parent in specs.__parents__: