Unable to convert generic case class to json using “writes”

Question

I have a class which I want to be able to convert to json:

case class Page[T](items: Seq[T], pageIndex: Int, pageSize: Int, totalCount: Long)

object Page {

  i         


        

Answer 1

I'd prefer this solution with trait, but in case you do want to make your case class generic you could use one of 2 approaches.

In case you don't have to use Page[_], i.e. you'll always call toJson on Page[Int] or Seq[Page[String]], but not on Page[_] or Seq[Page[_]]:

object Page {
  implicit def pageWriter[T: Writes](): Writes[Page[T]] = Json.writes[Page[T]]
}

In case you have to serialize Page[_]:

case class Page[T](items: Seq[T],
                   pageIndex: Int,
                   pageSize: Int,
                   totalCount: Long)(
      implicit val tWrites: Writes[T])

object Page {
  implicit def pageWriter[T]: Writes[Page[T]] = new Writes[Page[T]] {
    def writes(o: Page[T]): JsValue = {
      implicit val tWrites = o.tWrites
      val writes = Json.writes[Page[T]]
      writes.writes(o)
    }
  }
}