Play 2.1 Json serialization of traits - json

Question

The experimental "Inception" feature in Play 2.1 (Json.format[...]) only works for case classes (see here). How can I write my custom format implicit for a trait. I have the following construct:
sealed trait Plan {
def id: String
def name: String
def apps: Int
def users: Int
def testruns: Int
def price: Int
def prio: Int
}
And the following case classes which are extending the trait Plan.
case class Start(
id: String = "start",
name: String = "Start",
apps: Int = 1,
users: Int = 1,
testruns: Int = 10,
price: Int = 99,
prio: Int = 30) extends Plan
case class Pro(
id: String = "pro",
name: String = "Pro",
apps: Int = 2,
users: Int = 5,
testruns: Int = 25,
price: Int = 299,
prio: Int = 20) extends Plan
case class Premium(
id: String = "premium",
name: String = "Premium",
apps: Int = -1,
users: Int = -1,
testruns: Int = -1,
price: Int = 799,
prio: Int = 10) extends Plan
Now I need to write my custom implicit format val in the Plan companion object. I tried:
object Plan {
implicit val planFormats = (
(__ \ "id").format[String] and
(__ \ "name").format[String] and
(__ \ "apps").format[Int] and
(__ \ "users").format[Int] and
(__ \ "testruns").format[Int] and
(__ \ "price").format[Int] and
(__ \ "prio").format[Int]
)(Plan.apply, unlift(Plan.unapply))
}
However, a trait hasn't an apply or unapply method. What is the correct way to provide an implicit val for the json serialization in Play 2.1?

Answer 1

You simply have to provide your own function that creates a new instance from the given values.
Essentially a companion object to the trait that acts as a factory.
object Plan {
def apply(id: String, name: String, ...) = id match {
case "pro" => new Pro(id, name, ...)
...
}
def unapply(p: Person): Option[(String, String, ...)] = ...
}

Answer 2

Why are you using Traits and implementing case class ?
Why not using instances of classes, such as:
case class Plan (
id: String,
name: String,
apps: Int,
users: Int,
testruns: Int,
price: Int,
prio: Int
)
val start = new Plan("start", "Start", 1, 1, 10, 99, 30)
val pro = new Plan("pro", "Pro", 2, 5, 25, 299, 20)
val premium = new Plan("premium", "Premium", -1, -1, -1, 799, 10)
and then, you can keep your Json formatter:
object Plan {
implicit val planFormats = (
(__ \ "id").format[String] and
(__ \ "name").format[String] and
(__ \ "apps").format[Int] and
(__ \ "users").format[Int] and
(__ \ "testruns").format[Int] and
(__ \ "price").format[Int] and
(__ \ "prio").format[Int]
)(Plan.apply, unlift(Plan.unapply))
}