Compiler error on nested futures response with Play and ReactiveMongo

Question

I have a mongo entry where one field is a list. If entry doesn't exist, I want to add new one. If it exists, I want to append a new element to the list.

At the end I want to return an Ok to the client, but only after the operation was completed successfully. Not a strict requirement, but it's what makes most sense for the user, in my opinion.

This is what I have currently - it works, but on update it overwrites the old list, instead of appending a new element.

def myMethod(value:String, value2:String) = Action {

  Async {
    val myElement = Json.obj(
      "key" -> value2
    )

    val myDBEntry = Json.obj(
      "key" -> value,
      "list" -> List(myElement)
    )

    collection.update(Json.obj("key" -> value), myDBEntry, upsert = true).map(lastError =>
      Ok("Mongo LastError: %s".format(lastError)))
  }
}

In order to check if the list exists and append the element / create a new list, I started with something like (this replaces the collection.update block in previous code):

val futureResult:Future[Option[JsObject]] = collection.find(Json.obj("key" -> value)).one[JsObject]

futureResult.map { result =>

  if (result.isEmpty) {
      collection.insert(myDBEntry).map(lastError =>
        Ok("Mongo LastError: %s".format(lastError)))


  } else {
      //this not the correct command yet - but compiler already fails because method is not returning correct future 
      collection.update(Json.obj("key" -> value), myDBEntry, upsert = true).map(lastError =>
        Ok("Mongo LastError: %s".format(lastError)))
  }


}

But the compiler doesn't seem to like this nesting: "Type mismatch, expected: Future[Result], actual: Future:[Future[SimpleResult[Nothing]]]"

Anyways I feel this way is a bit awkward, and there must be a more elegant way to solve this, but I'm new to Futures and ReactiveMongo and have no idea. How do I solve it?

Edit: I also found this post But I think that's returning the response before the DB operation is finished and I don't want that.

score 4 · Accepted Answer · answered Aug 13 '13 at 10:58

Try changing this line:

futureResult.map { result =>

To this:

futureResult.flatMap { result =>

When you are performing a map on a future and inside that map block are returning another future you need to use flatMap instead as it will flatten out the nesting.

Compiler error on nested futures response with Play and ReactiveMongo

1 Answers1