Laravel Eloquent HasOne::$id Undefined Property 错误的解决办法

Laravel Eloquent 报错：HasOne::$id Undefined Property，代码如下：

// Workflow 模型
class Workflow extends Model
{
    public function startNode(){
        return $this->hasOne(Node::class)->ofMany('sort', 'min');
    }
}

//调用报错
$start = $workflow->startNode()->id;

//正确调用方法，去掉方法后面的括号
$start = $workflow->startNode->id;