gistfile1.scala

controller
-------------
package com.example.app

class MyController extends ScalatraServlet with JacksonJsonSupport {

  implicit val jsonFormats = DefaultFormats + new UserSerializer

  before() {
    contentType = formats("json")
  }

  get("/") {
    User.find
  }
  
}

Model
----------
package com.example.app

case class User (id: Int, email: String)

object User {

  def find = { new User(1,"Adam")}

}


Serializer
-------------------
package com.example.app

class UserSerializer extends CustomSerializer[User](format => ({
  case JObject(
  ("id", JString(id)) ::
    ("email", JString(email)) :: Nil) =>
    new User(2,"Adam2")
}, {
  case user: User =>
    JObject.apply(
      "email" -> JString(user.email),
      "id" -> JInt(user.id) )
    }))