code: def find_one_by_id(self, _id):
          
          document = (yield self.collection.find_one({"_id": ObjectId(_id)}))
          raise Return(self._obj_cursor_to_dictionary(document))
docs: Find a single document by id
  
          :param str _id: BSON string repreentation of the Id
          :return: a signle object
          :rtype: dict