Help me write a Play Json format for generic (bounded types) class

[Noor Rayni]

I am trying to write a play Json formatter for the enum and classes below. I want to de-serialize a Json file to an instance of ImageDisplay (see below). When I run the following code, Play complains that I cannot find a deserializer for ImageDisplay[T]. I have written different formats but none of them actually worked. Please help.

environment.resourceAsStream("cdn.images.json").flatMap { is =>
      try {
        //          play.api.libs.json.Json.parse(IOUtils.toString(is)).as[ImageDisplay[T]]
        Json.fromJson[models.ImageDisplay[T]](Json.parse(IOUtils.toByteArray(is))).asOpt
      } finally {
        is.close()
      }
    }

object Enum extends Enumeration {
  val Primary = Value("primary", "0")
  case class Category(name: String, val key: String) extends Val(nextId, name)
  protected final def Value(name: String, key: String): Category = new Category(name, key)

}

case class BaseImage(category: Enum.Category, url: String, height: Int, width: Int, units: String, format: String)

abstract class ImageSet

case class ProductImageSet(swatchImage: BaseImage, smallImage: BaseImage, mediumImage: BaseImage, largeImage: BaseImage) extends ImageSet

case class ProfileImageSet(smallImage: BaseImage, mediumImage: BaseImage) extends ImageSet

case class ImageDisplay[T <: ImageSet](id: String, imageSets: Seq[T])

[Will Sargent]

I don't think you can use case classes with type parameters with JSON macros.  You may need to specify concrete instances.

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[Greg Methvin]

Hi Noor,

As Will said, it's not possible to use any of the JSON macros for this. You're going to have to write it manually. Also, you probably need an implicit Format[T] in your ImageDisplay, since to generate a Format for ImageDisplay you also need a Format for whatever type it contains.

You'll have to roll your own serializer for your Enumeration too, but for each of your ImageSets you can still use the Json.format macro.

Greg

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Greg Methvin

Senior Software Engineer

Lightbend, Inc.

[Noor Rayni]

Thanks Greg and Will for the answers. Appreciate the answers. Actually, I have defined some implicit Format[T] but Play still complains it cannot find a deserializer for ImageDisplay[T]. Here are the implicit formats I defined for my classes. Any suggestions please ?

object ProfileImageSet {

  implicit def profileImageSetFormat: Format[ProfileImageSet] = (
    (__ \ "smallImage").format[BaseImage] ~
    (__ \ "mediumImage").format[BaseImage])(ProfileImageSet.apply _, unlift(ProfileImageSet.unapply _))
}

object ProductImageSet {
  implicit def productImageSetFormat: Format[ProductImageSet] = (
    (__ \ "swatchImage").format[BaseImage] ~
    (__ \ "smallImage").format[BaseImage] ~
    (__ \ "mediumImage").format[BaseImage] ~
    (__ \ "largeImage").format[BaseImage])(ProductImageSet.apply _, unlift(ProductImageSet.unapply _))

}

object BaseImage {
  implicit def baseImageFormat: Format[BaseImage] = (
    (__ \ "category").format[Enum.Category] ~
    (__ \ "url").format[String] ~
    (__ \ "height").format[Int] ~
    (__ \ "width").format[Int] ~
    (__ \ "units").format[String] ~
    (__ \ "format").format[String])(BaseImage.apply, unlift(BaseImage.unapply))
}

object ImageDisplay {
  implicit def imageDisplayFormat[T: Format]: Format[ImageDisplay[T]] =
    ((__ \ "pin").format[String] ~
      (__ \ "imageSets").format[Seq[T]])(ImageDisplay.apply _, unlift(ImageDisplay.unapply _))

  implicit def profileImageDisplayFormat[T: Format]: Format[ImageDisplay[ProfileImageSet]] =
    ((__ \ "pin").format[String] ~
      (__ \ "imageSets").format[Seq[ProfileImageSet]])(ImageDisplay.apply _, unlift(ImageDisplay.unapply _))

  implicit def productImageDisplayFormat[T: Format]: Format[ImageDisplay[ProductImageSet]] =
    ((__ \ "pin").format[String] ~
      (__ \ "imageSets").format[Seq[ProductImageSet]])(ImageDisplay.apply _, unlift(ImageDisplay.unapply _))
}

[Noor Rayni]

Okay, I got it to work by having two services, on for each ImageSet, to parse my Jsons. I am duplicating code but at least got the de-serialization to work.