Laravel 如何获取相关表的摘要？

在laravel 7应用程序中，我有两个与UMThread相关的表：

Schema::create('forum_threads', function (Blueprint $table) {
    $table->increments('id')->unsigned();
    ...
    $table->integer('forum_id')->unsigned();
    $table->foreign('forum_id')->references('id')->on('forums')->onUpdate('RESTRICT')->onDelete('CASCADE');

});

对于UMPOST：

Schema::create('forum_posts', function (Blueprint $table) {
    $table->bigIncrements('id')->unsigned();
    ...
    $table->integer('forum_thread_id')->unsigned();
    $table->foreign('forum_thread_id')->references('id')->on('forum_threads')->onUpdate('RESTRICT')->onDelete('CASCADE');
    ...
    $table->integer('viewed')->unsigned()->default(0);

});

最后一个字段已查看，我需要计算相关论坛文章行的已查看字段的总和。 在模型app/ForumThread.php中，我添加了forumPostsViewedSum方法：

    public function forumPosts()
    {
        return $this->hasMany('App\ForumPost');
    }

    public function forumPostsViewedSum()
    {
        return $this->hasMany('App\ForumPost')
                    ->selectRaw('SUM(viewed) as forum_posts_viewed_sum')
                    ->groupBy('forum_thread_id');
//                    ->groupBy('forum_id');
    }

在控制器中：

$forumThreads = ForumThread
    ::with('latestForumPost.creator')
    ->withCount('forumPosts')
    ->with('creator')
    ->getByForumId($forum_id)
    ->orderBy($order_by, $order_direction)
    ->offset($limit_start)
    ->take($forum_threads_per_page)
    ->with('forumPostsViewedSum')
    ->get();
\Log::info(varDump($forumThreads, '  get_forum_threads  -3 $forumThreads::'));

通过查看跟踪sql，我知道我需要什么

   SELECT * 
    FROM `vfrm_forum_posts` 
    WHERE `vfrm_forum_posts`.`forum_thread_id` in (2, 3) 
    ORDER BY `created_at` desc 
...  
  
  
   SELECT SUM(viewed)     AS forum_posts_viewed_sum 
    FROM `vfrm_forum_posts` 
    WHERE `vfrm_forum_posts`.`forum_thread_id` in (2, 3) 
    GROUP BY `forum_thread_id` 

上一个请求返回了我期望的数据，但在返回的数据和跟踪的数据（Log:：info）中，我得到了论坛帖子的空数组

为什么论坛帖子的数据无效？我怎么做

谢谢