def relation_column(instance, fields):
    '''
    such as: user.username
    such as: replies.content
    '''
    relation = getattr(instance.__class__, fields[0]).property
    _field = getattr(instance, fields[0])
    if relation.lazy == 'dynamic':
        _field = _field.first()
    return getattr(_field, fields[1]) if _field else ''
such as: user.username
    such as: replies.content