public class ApartmentService {
   private static final ApartmentId NO_ID = ApartmentId.nullObject();

   public ApartmentId add(OwnerId ownerId, ApartmentDto dto) {
       if (ownerRepository.exists(ownerId)) {
           Address address = new Address(dto.getStreet(), dto.getHouseNumber(), dto.getApartmentNumber(), dto.getCity(), dto.getCountry());
           Optional<Apartment> found = apartmentRepository.findBy(address);

           if (found.isPresent()) {
               Apartment apartment = found.get();

               eventRegistry.publish(new ApartmentAlreadyCreated(apartment.getId(), dto.getStreet(), dto.getHouseNumber(), dto.getApartmentNumber(), dto.getCity(), dto.getCountry()));
               return apartment.getId();
           } else {
               Apartment apartment = new Apartment(ownerId, address);
               return apartmentRepository.save(apartment);
           }
       } else {
           eventRegistry.publish(new OwnerNotFound(ownerId));
       }

       return NO_ID;
   }