Scala Play: List to Json-Array

Question

I've got a List which holds some Personalization-objects. The latter is defined like this:

  sealed case class Personalization(firstname: String, lastname: String, keycardId: String)

I need to map this list to a Json-Array structure which has to look like this:

"personalization": [
    {
      "id": "firstname",
      "value": "John"
    },
    {
      "id": "lastname",
      "value": "Doe"
    }...

I am struggling with the part of mapping the field information to id/value pairs. Normally, I would create a play.api.libs.json.Format out of the Personalization class and let it map automatically -> Json.format[Personalization] - but this time, I need to create an array where an entry can hold n attributes.

Therefore I am asking for advice, if there is a possibility to use the Scala Play-Framework? Any input is much appreciated, thank you!

cchantep · Accepted Answer · 2020-11-13 12:57:09Z

Writing as such JSON representation is not quite complex, using Writes.transform.

import play.api.libs.json._

case class Personalization(firstname: String, lastname: String, keycardId: String) // No need to seal a case class

implicit def writes: Writes[Personalization] = {
  val tx: JsValue => JsValue = {
    case JsObject(fields) => Json.toJson(fields.map {
      case (k, v) => Json.obj("id" -> k, "value" -> v)
    })

    case jsUnexpected => jsUnexpected // doesn't happen with OWrites
  }

  Json.writes[Personalization].transform(tx)
}

Which can be tested as bellow.

val personalization = Personalization(
  firstname = "First",
  lastname = "Last",
  keycardId = "Foo")

val jsonRepr = Json.toJson(personalization)
// => [{"id":"firstname","value":"First"},{"id":"lastname","value":"Last"},{"id":"keycardId","value":"Foo"}]

Reading is a little bit tricky:

implicit def reads: Reads[Personalization] = {
  type Field = (String, Json.JsValueWrapper)

  val fieldReads = Reads.seq(Reads[Field] { js =>
    for {
      id <- (js \ "id").validate[String]
      v <- (js \ "value").validate[JsValue]
    } yield id -> v
  })

  val underlying = Json.reads[Personalization]

  Reads[Personalization] { js =>
    js.validate(fieldReads).flatMap { fields =>
      Json.obj(fields: _*).validate(underlying)
    }
  }
}

Which can be tested as bellow.

Json.parse("""[
  {"id":"firstname","value":"First"},
  {"id":"lastname","value":"Last"},
  {"id":"keycardId","value":"Foo"}
]""").validate[Personalization]
// => JsSuccess(Personalization(First,Last,Foo),)

Note that is approach can be used for any case class format.

Oleg Zinoviev · Accepted Answer · 2020-11-13 11:33:04Z

0

Probably it is possible to do it a more elegant way I did, but you can use the following snippet:

case class Field(id: String, value: String)

object Field {
  implicit val fieldFormatter: Format[Field] = Json.format[Field]
}

sealed case class Personalization(firstname: String, lastname: String, keycardId: String)

object Personalization {

  implicit val personalizationFormatter: Format[Personalization] = new Format[Personalization] {
    override def reads(json:  JsValue): JsResult[Personalization] =
      Try {
        val data = (json \ "personalization").as[JsValue]
        data match {
          case JsArray(value) =>
            val fields = value.map(_.as[Field]).map(f => f.id -> f.value).toMap
            val firstname = fields.getOrElse("firstname", throw new IllegalArgumentException("Mandatory field firstname is absent."))
            val lastname = fields.getOrElse("lastname", throw new IllegalArgumentException("Mandatory field lastname is absent."))
            val keycardId = fields.getOrElse("keycardId", throw new IllegalArgumentException("Mandatory field keycardId is absent."))
            Personalization(firstname, lastname, keycardId)
          case _ => throw new IllegalArgumentException("Incorrect json format for Personalization.")
        }
      }.toEither.fold(e => JsError(e.getMessage), JsSuccess(_))

    override def writes(o:  Personalization): JsValue = {
      val fields = List(Field("firstname", o.firstname), Field("lastname", o.lastname), Field("keycardId", o.keycardId))
      JsObject(List("personalization" -> Json.toJson(fields)))
    }
  }
}

It converts {"personalization":[{"id":"firstname","value":"John"},{"id":"lastname","value":"Doe"},{"id":"keycardId","value":"1234"}]} to Personalization(John,Doe,1234) and vice versa

answered Nov 13, 2020 at 11:33

Oleg Zinoviev

5593 silver badges14 bronze badges

2 Comments

cchantep Over a year ago

Throwing exception, even in Try is not recommended, specially when JsResult could be used.

cchantep Over a year ago

.validate should be preferred to use .as

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