说我有一个这样的模型：

case class Items(typeOfItems: TypeOfItems)

object Items{
  implicit val format = Json.format[Items]
}


sealed trait TypeOfItems extends Product with Serializable

object TypeOfItems {

  final case class Toy(typeOfItem : String,name : String,price : Int) extends TypeOfItems

  final case class Car(typeOfItem : String,model: String,price : Int ) extends TypeOfItems

}

我无法进行任何序列化或反序列化。我收到以下错误：

 No instance of play.api.libs.json.Format is available for model.TypeOfItems in the implicit scope (Hint: if declared in the same file,make sure it's declared before)
[error]   implicit val format = Json.format[Items]
[error]                                    ^
[error]  No Json serializer found for type model.Items. Try to implement an implicit Writes or Format for this type.
[error]     Ok(Json.toJson(Items(Toy("Toy","Doll",2))))

我不确定如何为TypeOfItems指定格式。我怎么做？