Play Json framework can't parse json string if some field doesn't exist

Question

I defined a case class Fruit:

case class Fruit(name: String, weight: Double)

I wrote an action to save a fruit

  def saveFruit = Action(parse.json) { request: Request[JsValue] =>
    import models.Implicits._
    val json = request.body
    val fruitResult = Json.fromJson[Fruit](json)
    if (fruitResult.isSuccess) {
      val fruit = fruitResult.get
      println(fruit)
      Ok(JsString("1"))
    } else {
      Ok(JsString("2"))
    }

  }

If the request body is { "name":"ABC","weight":10}, then the action will be successfully called.

If the request body is { "name":"ABC"},then an error occurs,complaining that the weight is not undefined on object

{ "name":"ABC"} is a valid json string that can't parsed as a Fruit and weight will be null

I am not sure how to fix this issue.

I am using Play 2.6.0

looks like you want Fruit to use optional values? name: Option[String], weight: Option[Double]? — tgk
– tgk, Commented Jul 20, 2017 at 15:48
Thanks @tgk for the answer. I got another question stackoverflow.com/questions/45187140/…, could you please take a look? — Tom
– Tom, Commented Jul 20, 2017 at 22:57

rethab · Accepted Answer · 2017-07-20 16:39:29Z

1

I strongly suggest not going down the rabbit hole of allowing null anywhere. If you start allowing this in your request objects, you will never know whether there's a null anywhere in your models.

The typical way to approach this is to set this to Option as was suggested in the comments.

If you're working on a more elaborate solution, it might make sense to create dedicated request objects with lots of Options for values where the client might not send a value. You would then map/transform the request objects into your domain model in the controller and validate the input accordingly.

For example:

case class FruitRQ(name: String, weight: Option[Double])

case class Fuit(name: String, weight: Double)

class MyController {

  def myAction = {
    val fruitRq = Json.fromJson[FruiteRQ](request.body)
    (for (weight <- fruitRq.weight) yield {
      val fruit = Fruit(fruitRq.name, weight)
      // do something with the validate fruit
    }).getOrElse {
      // return 4xx to client saying that a field is missing
    }
  }
}

answered Jul 20, 2017 at 16:39

rethab

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1 Comment

Tom Over a year ago

Thanks @rethab for the answer. I got another question stackoverflow.com/questions/45187140/…, could you please take a look?

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