Play Scala Read/Write depending on type

Question

i have trouble dealing with Json reader in play framework 2.3 depending on a class field :

My model :

trait Data{
  val id:String
  val type:String
  val creation_date:Date
}

case class Price(
  id:String,
  type:String,
  creation_date:Date,
  amount:Int,
  currency:String) extends Data

case class Weight(
  id:String,
  type:String,
  creation_date:Date,
  value:Int,
  unit_of_measurement:String) extends Data

case class MyObject(
  id:String,
  data:List[Data]
)

My problem is : How can i read MyObject class and keep all values from Json like :

{
  id:"1",data:
    [
      {
        id:"1",
        type:"price",
        creation_date:"2014-12-01T00:00:00.000Z",
        amount:99,
        currency:"dollar"
      },
      {
        id:"1",
        type:"price",
        creation_date:"2014-12-01T00:00:00.000Z",
        amount:99,
        currency:"dollar"
      },
      {
        id:"1",
        type:"weight",
        creation_date:"2014-12-01T00:00:00.000Z",
        value:2,
        unit_of_measurement:"tonne"
      }
    ]
}

I tried this :

implicit val myObjectReads: Reads[MyObject] = (
  ....
  (__ \ "data").read(Reads.traversableReads[List,Data])

But it doesn't work

Many thanks for your help !

MC

Answer 1

You can use the orElse method in the Reads trait:

import play.api.libs.json.Reads.verifying

val priceReads: Reads[Price] = verifying[Price](_.`type` == "price")(Json.reads[Price])
val weightReads: Reads[Weight] = verifying[Weight](_.`type` == "weight")(Json.reads[Weight])
implicit val dataReads: Reads[Data] = priceReads.map(identity[Data]) orElse weightReads.map(identity[Data])
implicit val myObjectReads: Reads[MyObject] = Json.reads[MyObject]

Note that I needed to map the Reads[Price] and Reads[Weight] to Reads[Data] before using orElse because Reads is not covariant (a Reads[Price] is not a Reads[Data]). I also added the verifying reads to check the type value if both Price and Weight happened to have the same structure.