laravel报错讲非法标签设置或值为空？

Question

报错信息如下：
ErrorException in Factory.php line 159:
Illegal offset type in isset or empty
具体是：视图：

<p class="post-meta">
                            Posted on {{ $post->publish_at->format('F j  Y') }}
                            @if ($post->grades->count())
                                in
                                {!! join(', ', $post->gradeLinks()) !!}
                            @endif
                        </p>

模型model里的对应函数：

 public function gradeLinks($base = '/article_system?grade=%grade%')
    {
        $grades = $this->grades()->lists('grade');
        $return = [];
        foreach ($grades as $grade) {
            $url = str_replace('%grade%', urlencode($grade), $base);
            $return[] = '<a href="'.$url.'">'.e($grade).'</a>';
        }
        return $return;
    }

控制器：

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

        return view($layout, $data);

路由：

Route::get('/', function () {
    return redirect('/article_system');
});

Route::get('article_system', 'article_systemController@index');
Route::get('article_system/{slug}', 'article_systemController@showPost');

问题中的报错是在我点开对应gradelinks的那个标签时给的报错，另外，我写的那个任务队列Article_systemIndexData没有bug,在别的视图里用到这个job的地方都没报错，望大神指点迷津啊、、、、、orz

Answer 1

使用日期格式化时，需要先在model中设置dates属性：

protected $dates = ['publish_at'];

然后格式化这样写：

{{ $post->publish_at->format('F j, Y') }}

Answer 2

Grade::layout($grade)里返回值是什么样子