My PlayFramework Action returns before a Future is

I have a Scala PlayFramework function that calls MongoDB and gets a Future[Seq[Document]] result. After a map zoom/pan event, this Play Action function is called from JavaScript on a web page via xhttp/GET. My Action method on the Play side returns before the Future's onComplete/Success is executed. So I'm looking for a way to call a JavaScript function to get the data when the Scala Future's onComplete/Success fires. How would I do that, or am I looking at this wrong?

Here is the code in question.

def rect(swLon: Float, swLat: Float, neLon: Float, neLat: Float) = Action {
  val sb = new StringBuilder()
  sb.append("<tt>boundingBox: swLon=" + swLon + ", swLat=" + swLat + ", neLon=" + neLon + ", neLat=" + neLat + "</tt>")
  if (oDb.isDefined) {
    val collection: MongoCollection[Document] = oDb.get.getCollection(collectionName)
    val fut = getFutureOne(collection) // returns a Future[Seq[Document]]
    fut onComplete {
      case Success(docs) => { for (doc <- docs) { setMongoJson(doc.toJson } }
      case Failure(t) => { println("FAIL: " + t.getMessage) }
    }
  }
  Ok(sb.toString)
}

// below is temporary until I figure out a better way to store/return the result when it comes in
private var mongoJson: String = ""
private def setMongoJson(s: String): Unit = mongoJson = s

getFutureOne is temporary, it just does a db.collection.find().first().toFuture. I just wanted to make sure my connection to MongoDB was working, and it is. In actual fact I'll replace it with a query to return data that falls within the bounding box.

Action is not designed to work with futures. Use Action.async, which will "wait" (technically not wait, but schedule) for the future to finish:

def rect(swLon: Float, swLat: Float, neLon: Float, neLat: Float) = Action.async {
  val sb = new StringBuilder()
  sb.append("<tt>boundingBox: swLon=" + swLon + ", swLat=" + swLat + ", neLon=" + neLon + ", neLat=" + neLat + "</tt>")
  if (oDb.isDefined) {
    val collection: MongoCollection[Document] = oDb.get.getCollection(collectionName)
    val fut = getFutureOne(collection) // returns a Future[Seq[Document]]
    fut.map {docs => 
      setMongoJson(doc.toJson)
      Ok(sb.toString)
    } recover {
      case e => BadRequest("FAIL: " + e.getMessage)
    }
  } else Future.successful(Ok("Not defined"))
}

Take a look at this for reference: https://www.playframework.com/documentation/2.4.x/ScalaAsync