r2dbc ItemService update

ItemService.java

@Service
@RequiredArgsConstructor
public class ItemService {

    private final ItemRepository itemRepository;
    private final ItemTagRepository itemTagRepository;
    private final TagMapper tagMapper;

    /**
     * Update an Item
     * @param itemToSave item to be saved
     * @return the saved item without the related entities
     */
    @Transactional
    public Mono<Item> update(Item itemToSave) {

        if(itemToSave.getId() == null || itemToSave.getVersion() == null) {
            return Mono.error(new IllegalArgumentException("When updating an item, the id and the version must be provided"));
        }

        return verifyExistence(itemToSave.getId())

                // Find the existing link to the tags
               .then(itemTagRepository.findAllByItemId(itemToSave.getId()).collectList())

               // Remove and add the links to the tags
               .flatMap(currentItemTags -> {

                   // As R2DBC does not support embedded IDs, the ItemTag entity has a technical key
                   // We can't just replace all ItemTags, we need to generate the proper insert/delete statements

                   final Collection<Long> existingTagIds = tagMapper.extractTagIdsFromItemTags(currentItemTags);
                   final Collection<Long> tagIdsToSave = tagMapper.extractTagIdsFromTags(itemToSave.getTags());

                   // Item Tags to be deleted
                   final Collection<ItemTag> removedItemTags = currentItemTags.stream()
                           .filter(itemTag -> !tagIdsToSave.contains(itemTag.getTagId()))
                           .collect(Collectors.toList());

                   // Item Tags to be inserted
                   final Collection<ItemTag> addedItemTags = tagIdsToSave.stream()
                           .filter(tagId -> !existingTagIds.contains(tagId))
                           .map(tagId -> new ItemTag(itemToSave.getId(), tagId))
                           .collect(Collectors.toList());

                   return itemTagRepository.deleteAll(removedItemTags)
                           .then(itemTagRepository.saveAll(addedItemTags).collectList());
               })

               // Save the item
               .then(itemRepository.save(itemToSave));
    }

    ...

    private Mono<Boolean> verifyExistence(Long id) {
        return itemRepository.existsById(id).handle((exists, sink) -> {
            if (Boolean.FALSE.equals(exists)) {
                sink.error(new ItemNotFoundException(id));
            } else {
                sink.next(exists);
            }
        });
    }
  
    ...

}