code: def find_one(self, **kwargs):
          
          future = TracebackFuture()
  
          def handle_response(result, error):
              if error:
                  future.set_exception(error)
              else:
                  instance = self.__entity()
                  instance.map_dict(result)
                  future.set_result(instance)
  
          self.__collection.find_one(kwargs, callback=handle_response)
  
          return future
docs: Returns future.
  
          Executes collection's find_one method based on keyword args
          maps result ( dict to instance ) and return future
  
          Example::
  
             manager = EntityManager(Product)
             product_saved = yield manager.find_one(_id=object_id)