How to write a #Json #Reads which replaces a key by another one and prunes the original branch ? #play2.1 #scala

solution.scala

val reads = __.json.update( // copies the full JSON
    (__ \ "_id").json.copyFrom( (__ \ 'id).json.pick ) // adds _id
) andThen (__ \ "id").json.prune)  // and after removes id