Scala Play Read: How to Flatten Json containing Array of arrays to model

Question

I am looking for a way to define a Reads which allows me to map a JSON containing the following structure:

{
    "offers": [
        [
            {
                "id": "1234",
                (etc.)
            }
        ]
    ]
}

to model such case class TransportOffer(offers: List[Offer])

Unfortunately I haven't been able to do this yet. This is my code:

implicit val transportOfferReads: Reads[TransportOffer] = (
    (JsPath \ "offers").read[List[List[Offer]]].flatMap(r => r.flatten)
    )(TransportOffer.apply _)

In this case the flattening is not possible, as flatMap expects another Reads. How would I wrap the flattened List into another Reads? Or is there a simpler way?

Tomer Shetah · Accepted Answer · 2021-01-27 09:52:42Z

1

I'll present 3 options:

Flattening in a short reads:

case class Offer(id: String)

object Offer {
  implicit val format: OFormat[Offer] = Json.format[Offer]
}

case class TransportOffer(offers: List[Offer])

object TransportOffer {
  implicit val transportOfferReads: Reads[TransportOffer] =
    (JsPath \ "offers").read[List[List[Offer]]].map(x => TransportOffer(x.flatten))
}

Then calling:

Json.parse(jsonString).validate[TransportOffer].foreach(println)

outputs:

TransportOffer(List(Offer(1234)))

Code run at Scastie

Explicitly writing Reads:

implicit val transportOfferReads: Reads[TransportOffer] = (json: JsValue) => {
  json \ "offers" match {
    case JsUndefined() =>
      JsError("offers undefined")
    case JsDefined(value) =>
      value.validate[List[List[Offer]]].map(x => TransportOffer(x.flatten))
  }

Code run at Scastie.

First transform the json, into the model you'd like. For that define a transformer:

val jsonTransformer = (__ \ "offers").json
  .update(__.read[JsArray].map{o => {
    JsArray(o.value.flatMap(_.asOpt[JsArray].map(_.value)).flatten)
  }})

Then, assuming we have the case classes and their formatters:

case class Offer(id: String)

object Offer {
  implicit val format: OFormat[Offer] = Json.format[Offer]
}

case class TransportOffer(offers: List[Offer])

object TransportOffer {
  implicit val format: OFormat[TransportOffer] = Json.format[TransportOffer]
}

We can call:

Json.parse(jsonString).transform(jsonTransformer) match {
  case JsSuccess(value, _) =>
    value.validate[TransportOffer].foreach(println)
  case JsError(errors) =>
    println(errors)
    ???
}

Output is:

TransportOffer(List(Offer(1234)))

Code run at Scastie.

edited Jan 27, 2021 at 9:52

answered Jan 26, 2021 at 21:56

Tomer Shetah

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3 Comments

Remo Over a year ago

Thanks a lot @TomerShetah for your input. Just to clarify - is there no possibility to flatten the JsArray on Reads?

Remo Over a year ago

@TomerShetah well that would work when TransportOffer only takes one parameter right?

Tomer Shetah Over a year ago

Yes, for more parameters you need to use the and operator

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