@classmethod
def get_disabled_fields(cls, obj, ar):
    """
        Return the cached set of disabled fields for this `obj` and `ar`.

        """
    df = getattr(obj, '_disabled_fields', None)
    if df is None:
        df = cls.make_disabled_fields(obj, ar)
        setattr(obj, '_disabled_fields', df)
    return df
