Laravel Nova - relation fields

Customer.php

// Customer hasOne or belongsTo User

public function fields(Request $request)
{
  return [
      Text::make('Heslo', 'password')->onRelation('user'),
  ];
}

NovaServiceProvider.php

Field::macro('onRelation', function ($relation) {
    /** @var \Laravel\Nova\Fields\Field $this */
    return $this
        ->withMeta(['__relation' => $relation])
        ->resolveUsing(fn ($value, $model, $attribute) => 
            optional($model->$relation)->$attribute
        );
});

Resource.php

protected static function fillFields(NovaRequest $request, $model, $fields)
{
    $model::saved(function ($model) use ($request, $fields) {
        $relationFields = $fields
            ->filter(fn ($field) => isset($field->meta['__relation']))
            ->groupBy(fn ($field) => $field->meta['__relation']);

        foreach ($relationFields as $relation => $fields) {
            $relatedModel = $model->$relation()->firstOrNew();

            parent::fillFields($request, $relatedModel, $fields);

            $relatedModel->save();
        }
    });

    return parent::fillFields($request, $model, $fields->reject(fn ($field) => isset($field->meta['__relation'])));
}