Complete code for https://github.com/playframework/playframework/issues/2148.

debugging.scala

package models

import play.api.libs.functional.syntax._
import play.api.libs.json._
import play.api.libs.json.util._
import play.api.libs.json.Reads._
import play.api.data.validation.ValidationError

case class User(meta: Option[String], name: String, email: String)

object User {
  def apply(name: String, email: String) = new User(None, name, email)
}

trait UserReadsAndWrites {

  val userReads: Reads[User] = (
    (__ \ "name").read[String] ~
    (__ \ "email").read[String]
  )(User)

  // val userReads: Reads[User] = (
  //   (__ \ "name").read[String] ~
  //   (__ \ "email").read[String]
  // )(User.apply _)

  // val userReads: Reads[User] = (
  //   (__ \ "name").read[String] ~
  //   (__ \ "email").read[String]
  // )(User.apply(_: String, _: String))

  val userWrites: Writes[User] = (
    (__ \ "meta").writeOpt[String] ~
    (__ \ "name").write[String] ~
    (__ \ "email").write[String]
  )(unlift(User.unapply))

}