admin-panel laravel数组到字符串转换

Question

我正在使用http://laravel-admin.org/此扩展程序管理面板

我的控制器   class Related_productController扩展Controller     {         使用ModelForm;

    /**
     * Index interface.
     *
     * @return Content
     */
    public function index()
    {
        return Admin::content(function (Content $content) {

            $content->header('header');
            $content->description('description');

            $content->body($this->grid());
        });
    }

    /**
     * Edit interface.
     *
     * @param $id
     * @return Content
     */
    public function edit($id)
    {
        return Admin::content(function (Content $content) use ($id) {

            $content->header('header');
            $content->description('description');

            $content->body($this->form()->edit($id));
        });
    }

    /**
     * Create interface.
     *
     * @return Content
     */
    public function create()
    {
        return Admin::content(function (Content $content) {

            $content->header('header');
            $content->description('description');

            $content->body($this->form());
        });
    }

    /**
     * Make a grid builder.
     *
     * @return Grid
     */
    protected function grid()
    {
        return Admin::grid(Related_product::class, function (Grid $grid) {

            $grid->id('ID')->sortable();
            $grid->column('related_id');


        });
    }

    /**
     * Make a form builder.
     *
     * @return Form
     */
    protected function form()
    {
        return Admin::form(Related_product::class, function (Form $form) {

            $form->display('id', 'ID');
            $form->select('product_id')->options(product::all()->pluck('id', 'id'));     

               $form->multipleSelect('related_id')->options(product::all()->pluck('productname', 'productname'));


        });
    }
}

我的模特

class Related_product extends Model
{
   protected $table = 'related_product';
    public $timestamps = false;



public function setRelated_productAttribute($related_id)

 {

   if (is_array($related_id)) 
   {
        $this->attributes['related_id'] = implode(',', $related_id);

    }

}

当我选择multiselect并提交数据时，它显示错误，如
QueryException在Connection.php第664行中：

数组转换为字符串（SQL：插入related_product（product_id，related_id）值（48，））