docs: def build_mongo_doc(self, key_list):
          """
              Creates the components of aggregation pipeline
              :param key_list: list of key which will be used to create the components of aggregation pipeline
              :type  key_list: list
  
              :returns mongo_doc: dict
          """
          mongo_doc = {}
  
          if isinstance(key_list, list) and key_list:
  
              for key in key_list:
                  mongo_doc[key] = "$" + str(key)
          return mongo_doc