einundzwanzig-portal/app/Models/Library.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Library extends Model
{
    use HasFactory;

    /**
     * The attributes that aren't mass assignable.
     *
     * @var array
     */
    protected $guarded = [];

    /**
     * The attributes that should be cast to native types.
     *
     * @var array
     */
    protected $casts = [
        'id' => 'integer',
        'language_codes' => 'array',
    ];

    public function libraryItems(): \Illuminate\Database\Eloquent\Relations\BelongsToMany
    {
        return $this->belongsToMany(LibraryItem::class);
    }
}