code: def _collect_external_resources(self, resource_attr):
          
  
          external_resources = []
  
          for _, cls in sorted(Model.model_class_reverse_map.items(), key=lambda arg: arg[0]):
              external = getattr(cls, resource_attr, None)
  
              if isinstance(external, string_types):
                  if external not in external_resources:
                      external_resources.append(external)
              elif isinstance(external, list):
                  for e in external:
                      if e not in external_resources:
                          external_resources.append(e)
  
          return external_resources
docs: Collect external resources set on resource_attr attribute of all models.