Play! Scala: Json reader and additional class field

Question

I'm using Play! Scala 2.2 and I'm reading json as following and it works great:

case class YoutubeTrack(//artistId: String,
                          videoId: String,
                          title: String,
                          thumbnail: Option[String] )
val youtubeTrackReads: Reads[YoutubeTrack] = (
      (__ \ "id" \ "videoId").read[String] and
        (__ \ "snippet" \ "title").read[String] and
        (__ \ "snippet" \ "thumbnails" \ "default" \ "url").readNullable[String]
      )(YoutubeTrack)

Now I would like to add a field to my YoutubeTrack class (artistId that is commented in the class declaration). This field is a variable that I define somewhere else.

How can I add this field to my YoutubeTrack at the same time that I read the json i.e. I would like to do something like:

val youtubeTrackReads: Reads[YoutubeTrack] = (
      (__ \ "id" \ "videoId").read[String] and
        (__ \ "snippet" \ "title").read[String] and
        (__ \ "snippet" \ "thumbnails" \ "default" \ "url").readNullable[String]
      )((artistId, videoId, title, url) => YoutubeTrack(artistId, videoId, title, url))

Answer 1

Given function

def toArtistId(
    videoId: String,
    title: String,
    thumbnail: Option[String]): String = ...

1. You can even add your artist id without changing a reader

   case class YoutubeTrack(
       videoId: String,
       title: String,
       thumbnail: Option[String]) {
       val artistId = toArtistId(videoId, title, thumbnail)
   }
   

2. Or change your reader like this

   (
       (__ \ "id" \ "videoId").read[String] and 
       (__ \ "snippet" \ "title").read[String] and
       (__ \ "snippet" \ "thumbnails" \ "default" \ "url").readNullable[String]
   )((videoId, title, thumbnail) => 
        YoutubeTrack(toArtistId(videoId, title, thumbnail), videoId, title, thumbnail)
   )
   

3. Or even like this

   def artistReader(artistId: String): Reads[YoutubeTrack] = {
       (
           (__ \ "id" \ "videoId").read[String] and
           (__ \ "snippet" \ "title").read[String] and
           (__ \ "snippet" \ "thumbnails" \ "default" \ "url").readNullable[String]
       )((videoId, title, thumbnail) => 
           YoutubeTrack(artistId, videoId, title, thumbnail))
   }
   
   artistReader("A")