Php Symfony 3通过外键进行的条令索引

为什么这样做有效：

$query = $this->_em->createQuery(
            'SELECT ua.id, ua.correct, (ua.user) as user_id
            FROM AppBundle:UserAnswer ua
            INDEX BY ua.id'
        );

但是，当我尝试按相关实体用户编制索引时，它不再起作用：

$query = $this->_em->createQuery(
            'SELECT ua
            FROM AppBundle:UserAnswer ua
            INDEX BY ua.user'
        );

出现错误：

[2/2] QueryException: [Semantical Error] line 0, col 83 near 'user': Error: Invalid PathExpression. Must be a StateFieldPathExpression.   +

[1/2] QueryException: SELECT ua
FROM AppBundle:UserAnswer ua
INDEX BY ua.user   +

改为：

$query = $this->_em->createQuery(
            'SELECT ua.id, ua.correct, (ua.user) as user_id
            FROM AppBundle:UserAnswer ua
            INDEX BY (ua.user)'
        );

抛出一个错误：

[2/2] QueryException: [Syntax Error] line 0, col 126: Error: Expected end of string, got '('   +
[1/2] QueryException: SELECT ua.id, ua.correct, (ua.user) as user_id
FROM AppBundle:UserAnswer ua
INDEX BY IDENTITY (ua.user)   +

如何将键作为外键对结果数组进行索引