How Can I Merge a Single<List<List<T>>> Into a List<T> with RxJava 2? - kotlin

Question

What I want to do is hit an endpoint to get a list of users, that returns Single<List<User>>. Next, I want to grab the first three users and hit another endpoint to get all of their posts Single<List<Post>>. Finally I want to display a Toast that has the total number of posts for all of the first 3 users.
I've been able to achieve it with the flatten() function available in Kotlin. However, I'd like to know how to do this using only RxJava 2. Is it possible? Thanks.
...
getPostsForFirstThreeUsers()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{ posts -> toast("There are: ${posts.flatten().size} posts") },
{ ex -> Timber.e(ex, "there was an error processing the request") }
)
fun getFirstThreeUsers(): Flowable<User> {
return getAllUsers()
.flattenAsFlowable { users -> users }
.doOnNext { Timber.i("a user: ${it.username}") }
.take(3)
}
fun getPostsForFirstThreeUsers(): Single<List<List<Post>>> {
return getFirstThreeUsers()
.flatMapSingle { api.getUsersPosts(it.id) }
.doOnNext { Timber.i("number of posts: ${it.size}") }
.toList()
}

Answer 1

Since you're only looking for the total number of posts, you could just flatmap the lists to a stream and then count. The flat-mapping will not guarantee order, but flattens the list of list into a single stream.
getPostsForFirstThreeUsers()
.subscribeOn(Schedulers.io())
.flatmapObservable(Observable::fromIterable)
.flatmap(Observable::fromIterable)
.count()
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{ count -> toast("There are: ${count} posts") },
{ ex -> Timber.e(ex, "there was an error processing the request") }
)

Answer 2

I haven't tested this but I think you can do this with flatmap and reduce (or collect). probably something's like:
getAllUsers()
.take(3)
.flatMap { api.getUsersPosts(it.id) }
.map { it.size }
.reduce(0) { acc, e -> acc + e }
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{ posts -> toast("There are: ${posts.flatten().size} posts") },
{ ex -> Timber.e(ex, "there was an error processing the request") }
)

Answer 3

I ended up going with this solution, which allows me to have access to the entire List of Post objects. Thanks for the direction.
fun getPostsForFirstThreeUsers(): Single<MutableList<Post>> {
return getFirstThreeUsers()
.flatMapSingle { api.getUsersPosts(it.id) }
.doOnNext { Timber.i("number of posts: ${it.size}") }
.flatMapIterable { it }
.doOnNext { Timber.i("the post: $it") }
.toList()
}