docs: def source_attr(attr_name):
      """
      Creates a getter that will drop the current value
      and retrieve the source's attribute with specified name.
      @param attr_name: the name of an attribute belonging to the source.
      @type attr_name: str
      """
  
      def source_attr(_value, context, **_params):
          value = getattr(context["model"].source, attr_name)
          return _attr(value)
  
      return source_attr