How to transform a JSON field into a composed class member during serialization.

1_Password.scala

// This is just a basic sealed trait implementing a couple cases.
sealed trait Password {
  val value: String
  def hashed: HashedPassword
  def check(plainTextCandidate: String): Boolean
}

case class PlainTextPassword(value: String) extends Password {
  def hashed = HashedPassword.Scrypt(value)
  def check(plainTextCandidate: String) = plainTextCandidate == value
}

case class HashedPassword(value: String) extends Password {
  def hashed = this
  def check(plainTextCandidate: String) = Scrypt.check(value, plainTextCandidate)
}

// And here we have our serialization. It's pretty straight-forward since even though
// we need to manipulate the class and value at serialization/deserialization,
// it's a 1:1 mapping from the class to JSON, and the field name and class names are
// the same.
object Password extends (String => PlainTextPassword) {
  def apply(plainText: String) = PlainTextPassword(plainText)
  
  private[this] val read: Formats => PartialFunction[JValue, Password] = formats => { 
    case JString(value) => HashedPassword(value)
  }

  private[this] val write: Formats => PartialFunction[Any, JValue] = formats => {
    case HashedPassword(value) => JString(value)
    case password @ PlainTextPassword(_) => JString(password.hashed.value)
  }
 
  object Format extends CustomSerializer[Password](formats => (read(formats), write(formats)))
}

2_Classes.scala

// This is our Id type. Cloudant (CouchDB) returns _id and _rev members in
// the JSON that we want to compose into a single object to avoid "stringly typed" members.
sealed trait Id {
  val id: String
}

case class NewId(id: String) extends Id
case class IdWithRev(id: String, rev: String) extends Id

// Here we use our Password member. Which is fine. We've written a serializer for it,
// but we also use our Id here, which requires more work:
case class User(key: Id, email: String, password: Password)

// A default mapping would look like this:
case class User(_id: String, _rev: Option[String], password: Password)

3_Main.scala

implicit val formats = DefaultFormats + Password.Format

val json = ("_id", JString("user-1234")) ~ 
              ("_rev", JString("1-0abc2")) ~ 
              ("email", JString("me@example.com")) ~ 
              ("password", JString("$s0$e0801$CQdMqMrO2THZ4aa1eyPBew==$BEeFifVXtG/ju1y68Kxm/YkWFhyXoHrwjL15Pnbw06w="))

// NOTE: There is no "key" field in the JSON, so our formats will throw 
//   org.json4s.package$MappingException: No usable value for key
//   Can't convert JNothing to interface Id.
json.extract[User]