Consolidate a list of Futures into a Map in Scala - scala

Question

I have two case classes P(id: String, ...) and Q(id: String, ...), and two functions returning futures:
One that retrieves a list of objects given a list of id-s:
def retrieve(ids: Seq[String]): Future[Seq[P]] = Future { ... }
The length of the result might be shorter than the input, if not all id-s were found.
One that further transforms P to some other type Q:
def transform(p: P): Future[Q] = Future { ... }
What I would like in the end is, the following. Given ids: Seq[String], calculate a Future[Map[String, Option[Q]]].
Every id from ids should be a key in the map, with id -> Some(q) when it was retrieved successfully (ie. present in the result of retrieve) and also transformed successfully. Otherwise, the map should contain id -> None or Empty.
How can I achieve this?

Answer 1

Is there an .id property on P or Q? You would need one to create the map. Something like this?
for {
ps <- retrieve(ids)
qs <- Future.sequence(ps.map(p => transform(p))
} yield ids.map(id => id -> qs.find(_.id == id)).toMap
Keep in mind that Map[String,Option[X]] is usually not necessary, since if you have Map[String,X] the .get method on the map will give you an Option[X].

Answer 2

Edit: Now assumes that P has a member id that equals the original id-String, otherwise the connection between ids and ps gets lost after retrieve.
def consolidatedMap(ids: Seq[String]): Future[Map[String, Option[Q]]] = {
for {
ps <- retrieve(ids)
qOpts <- Future.traverse(ps){
p => transform(p).map(Option(_)).recover {
// TODO: don't sweep `Throwable` under the
// rug in your real code
case t: Throwable => None
}
}
} yield {
val qMap = (ps.map(_.id) zip qOpts).toMap
ids.map{ id => (id, qMap.getOrElse(id, None)) }.toMap
}
}
Builds an intermediate Map from retrieved Ps and transformed Qs, so that building of ids-to-q-Options map works in linear time.