private function mapCreatorAndChanger(Document $document, UserInterface $creator = null, UserInterface $changer = null)
    {
        $document->addField(
            $this->factory->createField('changer', $changer ? $changer->getUsername() : null, 'string')
        );
        $document->addField(
            $this->factory->createField('changer_id', $changer ? $changer->getId() : null, 'string')
        );

        $document->addField(
            $this->factory->createField('creator', $creator ? $creator->getUsername() : null, 'string')
        );
        $document->addField(
            $this->factory->createField('creator_id', $creator ? $creator->getId() : null, 'string')
        );
    }