Set mutator of Karyawan NIP

Karyawan.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
use Spatie\MediaLibrary\HasMedia;
use Spatie\MediaLibrary\InteractsWithMedia;

class Karyawan extends Model implements HasMedia
{
    use HasFactory, InteractsWithMedia;

    protected $table = 'karyawan';

    protected $fillable = [
        'user_id',
        'nip',
        'nama',
        'nik',
        'foto',
        'jenis_kelamin',
        'tempat_lahir',
        'tanggal_lahir',
        'alamat_domisili',
        'alamat_ktp',
        'no_hp',
        'bidang_id',
        'jabatan_id',
        'agama_id',
        'pendidikan_terakhir_id',
        'status_karyawan_id',
        'tanggal_masuk',
        'tanggal_keluar',
        'bank_id',
        'no_rekening',
    ];

    public function setNipAttribute()
    {
        // year join + birth date + 4 digit last karyawan id with zero padding

        $year = date('Y', strtotime($this->tanggal_masuk));
        $birth_date = date('Ymd', strtotime($this->tanggal_lahir));
        $gender = $this->attributes['jenis_kelamin']=='L' ? 1 : 2;
        $last_karyawan_id = Karyawan::count()+1;
        $increment = str_pad($last_karyawan_id, 4, '0', STR_PAD_LEFT);

        $this->attributes['nip'] = $year . $birth_date . $gender . $increment;
    }

    // boot function to set default value for setting nip attribute
    protected static function boot()
    {
        parent::boot();

        static::creating(function ($model) {
            $model->setNipAttribute();
        });
    }
}