scala trouble with play framework JSON serialization from case class

Question

Play 2.2.1, scala 2.10

// PersonModel.scala
case class PersonModel(name: String, age: Long)

object PersonModel2 {    

  implicit object PersonModelFormat extends Format[PersonModel] {
    def reads(json: JsValue): PersonModel = PersonModel(
      (json \ "name").as[String],
      (json \ "age").as[Long])
    def writes(u: PersonModel): JsValue = JsObject(List(
      "name" -> JsString(u.name),
      "age" -> JsNumber(u.age)))
  }

sbt says

[error] PersonModel.scala:15: overriding method reads in trait Reads of type (json: play.api.libs.json.JsValue)play.api.libs.json.JsResult[models.PersonModel];
[error]  method reads has incompatible type
[error]     def reads(json: JsValue): PersonModel = PersonModel(
[error]         ^

To explain why you get the error: the compiler is actually telling you that you are trying to override the method reads (defined in the Reads trait) returning a different type than the one defined. You should return a JsResult[PersonModel]. Hence, you can just wrap your call to the PersonModel constructor with a JsSuccess(PersonModel(..., ...)) and change the return type of the method! — jsirocchi
– jsirocchi, Commented Nov 22, 2013 at 10:05

jsirocchi · Accepted Answer · 2013-11-22 09:54:01Z

4

In this case, since you're not doing fancy things with the output json (like changing the key names in the resulting json object), I'd go for:

case class PersonModel(name: String, age: Long)

import play.api.libs.json._
implicit val personModelFormat = Json.format[PersonModel]

This way, you can, for example

scala> val j = PersonModel("julien", 35)
j: PersonModel = PersonModel(julien,35)
scala> println(Json.toJson(j))
{"name":"julien","age":35}

More info can be found here

HTH,

Julien

answered Nov 22, 2013 at 9:54

jsirocchi

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Comments

Faiz · Accepted Answer · 2013-11-22 05:43:49Z

2

Things have changed in recent versions, I'd say for the better. In 2.2.x, you'd do it this way, using the new functional syntax and combinators:

 import play.api.libs.json._
 import play.api.libs.functional.syntax._


 implicit val PersonModelFormat: Format[PersonModel] = (
     (__ \ "name").format[String] and
     (__ \ "age").format[Long]
 )(PersonModel.apply, unlift(PersonModel.unapply))

Much shorter!

The documentation for 2.2.x http://www.playframework.com/documentation/2.2.1/ScalaJsonCombinators provides a good explanation for the rationale for the change.

edited Nov 22, 2013 at 5:43

answered Nov 22, 2013 at 5:19

Faiz

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Comments

Juraj · Accepted Answer · 2016-06-06 12:29:06Z

0

For single usage there is an inline solution:

Json.writes[PersonModel].writes(personModelInstance)  // returns JsObject

From documentation:
macro-compiler replaces Json.writes[User] by injecting into compile chain the exact code you would write yourself

answered Jun 6, 2016 at 12:29

Juraj

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