how to combine two publisher in one in spring reactor - project-reactor

Question

I have implemented a dummy reactive repository but I am struggling with the update method:
#Override
public Mono<User> updateUser(int id, Mono<User> updateMono) {
return //todo with getUser
}
#Override
public Mono<User> getUser(int id) {
return Mono.justOrEmpty(this.users.get(id));
}
From one hand I have incoming publisher Mono<User> updateMono, from the other hand I have another publisher during Mono.justOrEmpty(this.users.get(id)).
How to combine it together, make update, and give back just one publisher?
The only thing come to my mind is:
#Override
public Mono<User> updateUser(int id, Mono<User> updateMono) {
return getUser(id).doOnNext(user -> {
updateMono.subscribe(update -> {
users.put(id, new User(id, update.getName(), update.getAge()));
System.out.format("Updated user with id %d to %s%n", id, update);
});
});
}
Is it correct?

Answer 1

See the reference guide on finding the right operator
Notably, for Mono you have and, when, then (note this last one will become flatMap in 3.1.0, and flatmap will become flatMapMany)
doOnNext is more for side operations like logging or stats gathering. Subscribe inside subscribe is another bad form; generally you want flatMap or similar instead.

Answer 2

I have played Spring 5 Reactive Streams features in these days, and have written down some sample codes(not public via blog or twitter yet, I still need more practice on Reactor).
I have encountered the same problems and finally used a Mono.zip to update the existing item in MongoDB.
https://github.com/hantsy/spring-reactive-sample/blob/master/boot-routes/src/main/java/com/example/demo/DemoApplication.java
public Mono<ServerResponse> update(ServerRequest req) {
return Mono
.zip(
(data) -> {
Post p = (Post) data[0];
Post p2 = (Post) data[1];
p.setTitle(p2.getTitle());
p.setContent(p2.getContent());
return p;
},
this.posts.findById(req.pathVariable("id")),
req.bodyToMono(Post.class)
)
.cast(Post.class)
.flatMap(post -> this.posts.save(post))
.flatMap(post -> ServerResponse.noContent().build());
}
Update: Another working version written in Kotlin.
fun update(req: ServerRequest): Mono<ServerResponse> {
return this.posts.findById(req.pathVariable("id"))
.and(req.bodyToMono(Post::class.java))
.map { it.t1.copy(title = it.t2.title, content = it.t2.content) }
.flatMap { this.posts.save(it) }
.flatMap { noContent().build() }
}