einundzwanzig-portal/app/Models/Library.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsTo;
use Illuminate\Database\Eloquent\Relations\BelongsToMany;
use Illuminate\Database\Eloquent\Relations\HasMany;

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',
    ];

    protected static function booted()
    {
        static::creating(function ($model) {
            if (!$model->created_by) {
                $model->created_by = auth()->id();
            }
        });
    }

    public function createdBy(): BelongsTo
    {
        return $this->belongsTo(User::class, 'created_by');
    }

    public function libraryItems(): BelongsToMany
    {
        return $this->belongsToMany(LibraryItem::class);
    }

    public function parent() : BelongsTo
    {
        return $this->belongsTo(__CLASS__, 'parent_id');
    }
}