From a MongoDB collection to Scala object array through Gson to JSON

I used the following code to convert an array of Scala objects on the server into a JSON string that I could return back to the client. So it's going from a MongoDB collection to an array of Scala objects through Gson to become a JSON string.

/**
 * Return a list of all stored sticky notes as a JSON string.
 */
get("/getStickyNotes") {
  var notes = ArrayBuffer[StickyNote]()
  val coll = getMongoDbCollection
  val dbObjects = coll.find
  for (dbObject <- dbObjects) {
    notes += convertDbObjectToStickyNote(dbObject)
  }
  // return array as json
  (new Gson).toJsonTree(notes.toArray)
}

def convertDbObjectToStickyNote(o: MongoDBObject): StickyNote = {
  val content = o.getAs[String]("content")
  val windowTitle = o.getAs[String]("windowTitle")
  val fontSizeIncrement = o.getAs[Int]("fontSizeIncrement")
  return StickyNote(content.get, windowTitle.get, fontSizeIncrement.get)
}

A few notes:

• The actual code is longer than this; I trimmed it down to make it easier to read here.
• I used gson.toJsonTree in this code, but I may have been able to use just gson.toJson. I started using toJsonTree because of an unrelated problem.
• If I was a cooler Scala programmer I'd probably use a collection method like flatMap instead of a for loop to create the notes object form dbObjects.
• Notice the need to use .get when I return the StickyNote in the second to last line of code. That's because I'm dealing with Some references.