How do I update a MongoDB document with new value using reactors Mono? (Kotlin) - mongodb

Question

So the context is that I require to update a value in a single document, I have a Mono, the parameter Object contains values such as username (to find the correct user by unique username) and an amount value.
The problem is that this value (due to other components of my application) is the value by which I need to increase/decrease the users balance, as opposed to passing a new balance. I intend to do this using two Monos where one finds the user, then this is combined to the other Mono with the inbound request, where I can then perform a simple sum (i.e balance + changeRequest.amount) then return this to the document database.
override fun increaseBalance(changeRequest: Mono<ChangeBalanceRequestResource>): Mono<ChangeBalanceResponse> {
val changeAmount: Mono<Decimal128> = changeRequest.map { it.transactionAmount }
val user: Mono<User> = changeRequest.flatMap { rxUserRepository.findByUsername(it.username)
val newBalace = user.map {
val r = changeAmount.block()
it.balance = sumBalance(it.balance!!, r!!)
rxUserRepository.save(it)
}
.flatMap { it }
.map { it.balance!! }
return Mono.just(ChangeBalanceResponse("success", newBalace.block()!!))
}
Obviously I'm trying to achieve this in a non-blocking fashion. I'm also open to using only a single Mono if that's possible/optimal. I also appreciate I've truly butchered the example and used .block as a placeholder to illustrate what I'm trying to achieve.
P.S this is my first post, so any tips on how to express my problem clearer would be useful.

Answer 1

Here's how I would do this in Java (Using Double instead of Decimal128):
public Mono<ChangeBalanceResponse> increaseBalance(Mono<ChangeBalanceRequestResource> changeRequest) {
Mono<Double> changeAmount = changeRequest.map(a -> a.transactionAmount());
Mono<User> user = changeRequest.map(a -> a.username()).flatMap(RxUserRepository::findByUsername);
return Mono.zip(changeAmount,user).flatMap(t2 -> {
Double changeAmount = t2.getT1();
User user = t2.getT2();
//assumes User is chained
return rxUserRepository.save(user.balance(sumBalance(changeAmount,user.balance())));
}).map(res -> new ChangeBalanceResponse("success",res.newBalance()))
}