laravel连表查询的问题？

Question

两张数据表：表一：grade_grade_pivot,有
id(key),father_grade_id,child_grade_id
三个键，
表二：grade ：有：title ，image_id, meta
想要查询父分级对应的子分级（在表一里），找到这些子分级后反馈出这些子分级的信息，
model,Grade中的关联如下：

 public function father_grade()
    {
        return $this->belongsToMany('App\Grade', 'grade_grade_pivot','child_grade_id','father_grade_id');
    }

控制器里：

 public function index(Request $request)
    {
        $grade = $request->get('grade');
        $data = $this->dispatch(new Article_systemIndexData($grade));
        $layout =  'article_system.layouts.index';

        return view($layout, $data);

    }

用到的jobs:Atricle_systemIndexData里的代码：

  protected function gradeIndexData($grade)
    {
        $grade = Grade::where('grade', $grade)->firstOrFail();
        $reverse_direction = (bool)$grade->reverse_direction;

     
        $gradesss = Grade::whereHas('father_grade',function ($q) use($grade) {
           //就是这里的代码该怎么写才好啊QAQ
           })；

        $page_image = $grade->page_image ?: config('blog.page_image');

        return [
            'title' => $grade->title,
            'subtitle' => $grade->subtitle,
            'page_image' => $page_image,
            'grade' => $grade,
            'reverse_direction' => $reverse_direction,
            'meta_description' => $grade->meta_description ?: config('blog.description'),
            'gradesss' => $gradesss
        ];
    }