course-site/app/Repositories/Models/Exam/Paper.php

<?php

namespace App\Repositories\Models\Exam;

use App\Repositories\Enums\ModelStatusEnum;
use App\Repositories\Models\Model;
use Prettus\Repository\Contracts\Transformable;
use Prettus\Repository\Traits\TransformableTrait;

/**
 * Class Paper.
 *
 * @package namespace App\Repositories\Models\Exam;
 */
class Paper extends Model implements Transformable
{
    use TransformableTrait;

    protected $table = 'exam_papers';
    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $guarded = [];

    public function topics()
    {
        return $this->belongsToMany(Topic::class, 'exam_paper_topic')->orderBy('sort', 'asc');
    }

    public function getAllotNumsAttribute()
    {
        return PaperResult::query()->where('paper_id', $this->attributes['id'])->where('status', ModelStatusEnum::OK)->where('is_answer', 0)->count();
    }
}