Hi Spring channel! I am having issue, building Rea...

# spring

[alieksie]

Hi Spring channel! I am having issue, building ReactiveEndpoint:

@PostMapping(SEARCH_ROUTE)
fun search(@RequestBody searchQuery: SearchQuery): Mono<SearchQuery> = searchService.save(searchQuery)

save(searchQuery) returns Mono<SearchQuery>.
When i return Mono.just(searchQuery), there are no issues. But when i return someMono.flatMap { ... }
I am getting an error: ...SearchQuery cannot be cast to class java.lang.String
Are there any tricks, or i miss something, when i use .flatMap ? 🙂

[Czar]

This isn't Kotlin related.
That said, your description is not enough to diagnose the problem. What's going on in the flatMap? And where are you trying to return the result of it? Are you returning it from SearchService#save(searchQuery: SearchQuery)?

[Czar]

In case of flatMap, if you want the result to be a Mono<Something>, you have to make sure that the result of the expression(s) between the curly braces of the flatMap to be Mono<Something>. If you do not want to construct Mono inside flatMap, use map instead.
E.g.

class SearchQuery
class SearchQueryPrecursor {
	fun toSearchQuery() : SearchQuery = SearchQuery()
}

class SearchService {
	fun save1(searchQuery: SearchQuery): Mono<SearchQuery> =
		Mono.just(SearchQueryPrecursor().toSearchQuery())
	
	fun save2(searchQuery: SearchQuery): Mono<SearchQuery> =
			Mono.just(SearchQueryPrecursor()).flatMap { Mono.just(it.toSearchQuery()) }
	
	fun save3(searchQuery: SearchQuery): Mono<SearchQuery> =
			Mono.just(SearchQueryPrecursor()).map { it.toSearchQuery() }
}

[alieksie]

Thank you for the Reply!
from the SearchService#save(searchQuery: SearchQuery)
i am returning:

// Mono<Boolean>
        return caches.searchQuery.store(cacheKey, preparedQuery).flatMap {
            result ->
                if (!result) {
                    Mono.error(ErrorSavingSearchQuery("Cannot save query: ${preparedQuery.buildQueryString()}"))
                } else {
                    Mono.just(preparedQuery)
                }
        }

[Czar]

I see, your problem is Mono.just(preparedQuery), where preparedQuery is probably a String, while you specified the expected result to be a Mono<SearchQuery>

[alieksie]

uh, preparedQuery is the instance of SearchQuery

[alieksie]

and also

[alieksie]

if i do something like return Mono.just(preparedQuery) before the flatMap, everything works fine 😕

[alieksie]

i also see, that flatMap returns MonoFlatMap object, maybe later internally something weird happens inside the Spring framework

[alieksie]

so i guess you are right, its not Kotlin related, have to dig deeper by myself 🙂

[Czar]

Sounds strange. I've tried to emulate your problem, but it works as expected for this small script:

import reactor.core.publisher.Mono

class SearchQuery {
	override fun toString(): String = "A Test"
	fun buildQueryString() = "ZZZ"
}

class ErrorSavingSearchQuery(message: String) : Throwable(message)

class SearchService {
	fun save(searchQuery: SearchQuery): Mono<SearchQuery> {
		val preparedQuery: SearchQuery = SearchQuery()
		return Mono.just(true).flatMap { result ->
			if (!result) {
				Mono.error(ErrorSavingSearchQuery("Cannot save query: ${preparedQuery.buildQueryString()}"))
			} else {
				Mono.just(preparedQuery)
			}
		}
	}
}

fun test() {
	val result: Mono<SearchQuery> = SearchService().save(searchQuery = SearchQuery())
	result.doOnSuccess {
		println("Hi, it's $it")
	}.subscribe()
}

test()

It prints

Hi, it's A Test

[alieksie]

i found an issue

[alieksie]

caches.searchQuery.store calls ReactiveRedisTemplate.opsForValue().set(), which actually returns MonoDefer

[Czar]

Ah, good 🙂

[alieksie]

where basically it should return Mono<Boolean> in the end

[alieksie]

Mono<Boolean> set(K key, V value);

[alieksie]

so just need to handle the MonoDefer case properly 😄

[alieksie]

but thank you for looking into it also!

[alieksie]

In the end it was issue of the proper serialisation of Object to Redis, hehe