//model 中
public function children()
    {
        return $this->hasMany(get_class($this), 'pid', 'id')
            ->with('children');
    }

    public static function list($limit)
    {
        $data = self::with('children')
            ->where('status', 1)
            ->where('pid', 0)
            ->latest('id');
        if ($limit) {
            return $data->paginate($limit);
        }
        return $data->get();
    }