Transform Mono<Void> in Mono<String> adding a value

Question

My program services offer some delete methods that return Mono<Void>, e.g.: fun delete(clientId: String) : Mono<Void>

After calling .delete("x") I would like to propagate the clientId downstream to do other operations:

userService.get(id).map{ user -> 
        userService.delete(user.id) //This returns Mono<Void>
    .map { 
        user.id //Never called!!!
    }
    .map { userId -> 
        //other calls using the propagated userId
    }
}

The problem is since delete returns a Mono<Void>, the following .map { user.id } is never called. So how can I transform the Mono<Void> into a Mono<String> to propagate the userId?

Answer 1

You can use thenReturn operator:

userService.get(id)
        .flatMap { user -> userService.delete(user.id).thenReturn(user.id) }
        .flatMap { id -> //other calls using the propagated userId }

Answer 2

I managed to work around it using hasNext that transforms it into a Boolean:

    @Test
    fun `should`() {
        val mono: Mono<String> = "1".toMono()
                .flatMap { id ->
                    val map = Mono.empty<Void>()
                            .hasElement()
                            .map {
                                id + "a"
                            }.map {
                                (it + "1")
                            }
                    map
                }


        mono.doOnNext {
            println(mono)
        }.subscribe()
    }