How to use return object of Mono without using block()?

Question

I am trying to learn spring webflux. In ReactiveMongoRepository, I am trying to check if category already exists. If it already exists then return that object otherwise save and return new saved object. Something like following.

public Mono<Category> save(Category category) {
    final Mono<Category> byId = repository.findById(category.getId());
    final Category block = byId.block();
    if (block == null) {
        return repository.save(new Category(category.getName()));
    } else {
        return byId;
    }
}

How can I do this without using block()?

Use switchIfEmtpy instead of block.

M. Deinum
– M. Deinum

2020-04-21 10:20:41 +00:00
Commented Apr 21, 2020 at 10:20 — M. Deinum
– M. Deinum, Commented Apr 21, 2020 at 10:20

Nikolas · Accepted Answer · 2020-04-21 12:53:38Z

1

Use Mono::switchIfEmpty that provides an alternative Mono in case the former one is completed without data. As long as ReactiveMongoRepository::save returns Mono, you can pass it to generate the alternative one.

return repository.findById(category.getId())
                 .switchIfEmpty(repository.save(new Category(category.getName())));

In case ReactiveMongoRepository::findById returns a Mono with data, the Mono::switchIfEmpty will not be called.

Edit: Using Mono::defer with a Supplier<Mono> makes the saving operation to be delayed when necessary:

.switchIfEmpty(Mono.defer(() -> repository.save(new Category(category.getName()))));

edited Apr 21, 2020 at 12:53

answered Apr 21, 2020 at 10:36

Nikolas

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2 Comments

Prashant Pandey Over a year ago

This is not totally correct. respository.save will be called eagerly.

Nikolas Over a year ago

Yes. The Mono with the result is returned immediately upon calling the save method and using Mono::defer should help. The question was about using avoiding block() operation. However, a good point!

Rajesh · Accepted Answer · 2020-04-21 10:47:23Z

1

You can try something like this

public Mono<Category> getCategories(Category category) {
   return repository.findByName(category.getName()).doOnNext(o -> {
   }).switchIfEmpty(repository.save(category));
}

edited Apr 21, 2020 at 10:47

answered Apr 21, 2020 at 10:39

Rajesh

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Comments

Prashant Pandey · Accepted Answer · 2020-04-21 12:35:26Z

0

You need to defer the switchIfEmpty. Otherwise, it'll be triggered eagerly:

return repository.findById(category.getId())
                 .switchIfEmpty(Mono.defer(() ->respository.save(category)));

answered Apr 21, 2020 at 12:35

Prashant Pandey

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