Having trouble getting Writes to work with Scala Play

Question

To begin with I would like to say sorry for long post, and I really appreciate those who still look into my problem.

I have a controller that should return a json-response with a structure like:

{
   result: [
       {
           key: value,
           key: value,
           key: value,
           key: [
              {
                 key: value,
                 key: value,
                 key: value
              },...
           ]
       },....
   ]
}

However I have problems getting the Writes to work as I want.

Note. I will add comments under the line where I have problem.

object APIController extends Controller {

  def feed() = Action {
    val objects = repo.getObjects().toList
    Ok(Json.toJson(Json.obj("result" -> Class_1.apply(objects).result)))
  }

first off, if I don't make a Json.obj("result" -> List[objects]) the result key isn't shown in the JSON-result. If I add a Writer for that I get errors saying that the List[objects] must have a Writer. But if I write it like above it doesn't need a Writer for the List[objects]

  case class Class_1 (result: Seq[Class_2])
  object Class_1 {
    def apply(objs: List[Object]) = {
      var result:ListBuffer[Class_2] = ListBuffer[Class_2]()
      for(obj <- objs) feedResult += Class_2.apply(code)
      new Class_1(result.toList)
    }
   }

*this is where I would put the Writer for Class_1. But if I do this like implicit val class1Writer = new Writes[Class_1] { override def writes(o: Class_1): JsValue = Json.obj("result" -> o.result) } I get the problems I mentioned earlier, that I suddenly need a Writes for a List[objects] of that type*

  case class Class_2 (id: Long, id2: Long, listOfStuff: Seq[Class_3])
  object Class_2 {
    def apply(obj: Object) = {
      var items: ListBuffer[Class_3] = ListBuffer[Class_3]()
      for(obj1 <- obj.getListOfStuff()) items += Class_3.apply(obj1)
      new Class_2(obj.firstID, obj.secID, items.toList)
    }
  }
  implicit val class2Writes = new Writes[Class_2] {
    override def writes(o: Class_2): JsValue = {
      Json.obj(
        "id" -> o.id,
        "id2" -> o.id2,
        "items" -> o.listOfStuff
      )
    }
  }

*the "items" -> o.listOfStuff says it needs a Writes for a List[types in the list] but I have a Writes for the objects in the list (Class_3) and I don't need a Writes for when serializing a list of objects from Class_2, why is it behaving like this?*

  case class Class_3 (id: Long, text: String)
  object Class_3 {
    def apply(obj: Object) = {
      new Class_3(obj.id, obj.funnyText)
    }
  }
  implicit val class3Writer = new Writes[Class_3] {
    override def writes(o: Class_3): JsValue = {
      Json.obj(
        "id" -> o.id,
        "text" -> o.text
      )
    }
   }
}

The error I get from this code is:

No Json deserializer found for type Seq[Class_3]. Try to implement an implicit Writes or Format for this type.
[error]         "items" -> o.listOfStuff
[error]                             ^

If I remove this line in the Writes it compiles and works. And I think that's weird since the first list I serialize doesn't have a Writer, only for the objects in the list.

Does anyone know why it behaves like this? What should I do to accomplish what I'm after? (I hope you see what I'm trying to do)

Thanks in advance.

Answer 1

Just put the implicit val class3Writer ahead of class2Writes