docs: def get_value(self, attributes,key = None):
          """Get value to be indexed from document attributes.
  
          :param attributes: Document attributes
          :type attributes: dict
          :return: Value to be indexed
          :rtype: object
  
          """
  
          value = attributes
          if key is None:
              key = self._splitted_key
  
          # A splitted key like 'a.b.c' goes into nested properties
          # and the value is retrieved recursively
          for i,elem in enumerate(key):
              if isinstance(value, (list,tuple)):
                  #if this is a list, we return all matching values for the given list items
                  return [self.get_value(v,key[i:]) for v in value]
              else:
                  value = value[elem]
  
          return value