Play Framework - Beginner Tutorial : How to handle a big json file in play ( more than 22 root variables) -

It is not a good practice to have such a big json with many root variables. Nevertheless, we might need to call a rest api that will give a json like this

{
  "a1": ...,
  "a2" : ...,
  "a3" : ...,
  "a4" : ...,
  "a5" : ...,
  "a6" : ...,
  "a7" : ...,
  "a8" : ...,
  "a9" : ...,
  "a10" : ...,
  "a11" : ...,
  "a12" : ...,
  "a13" : ...,
  "a14" : ...,
  "a15" : ...,
  "a16" : ...,
  "a17" : ...,
  "a18" : ...,
  "a19" : ...,
  "a20" : ...,
  "a21": ...,
  "a22" : ...,
  "a23" : ...,
  "a24" : ...,
  ....
}

First approach

Since scala 2.11 we can have case class with more than 22 fields, so the following compiles.

case class JsonExample (
    a1:String, a2:String, a3:String, a4:String, a5:String, a6:String,
    a7:String, a8:String, a9:String, a10:String, a11:String, a12:String,
    a13:String, a14:String, a15:String, a16:String, a17:String, a18:String,
    a19:String, a20:String, a21:String, a22:String, a23:String, a24:String
)

But if we try to use Play JSON automated mapping

import play.api.libs.json._
implicit val jsonExampleFormat = Json.format[JsonExample]

we get a compiler error

<console>:16: error: No unapply or unapplySeq function found
       implicit val jsonExampleFormat = Json.format[JsonExample]
                                                   ^

Instead we can use Play-Json extensions(requires play-json >= 2.4)

import ai.x.play.json.Jsonx
implicit val jsonExampleFormat = Jsonx.formatCaseClass[JsonExample]

jsonExampleFormat: play.api.libs.json.Format[JsonExample] = $anon$1@b7f7890

So now we have a Play Json format for a case class with more than 22 fields, and can use it as described in Play Json documentation

Second approach

I don’t consider having a case class with so many fields a good solution.

A better solution is to organize these fields in “logical” embedded case classes.

case class A (
    a1:String, a2:String, a3:String, a4:String,
    a5:String, a6:String, a7:String, a8:String
)

case class B (
    a9:String, a10:String, a11:String, a12:String,
    a13:String, a14:String, a15:String, a16:String
  )
case class C (
    a17:String, a18:String, a19:String, a20:String,
    a21:String, a22:String, a23:String, a24:String
  )
case class JsonExample2 (
    a : A,
    b : B,
    c : C
  )

We use a “regular” play json format for the embedded case classes.

import play.api.libs.json._
implicit val jsonAFormat : Format[A] = Json.format[A]
implicit val jsonBFormat : Format[B]= Json.format[B]
implicit val jsonCFormat : Format[C] = Json.format[C]

And the “trick” now is to use the JsPath Play Json Combinator for the embedded case classes without any path.

import play.api.libs.functional.syntax._
implicit val jsonExample2Format: Format[JsonExample2]  = (
  (JsPath).format[A] and
  (JsPath).format[B] and
  (JsPath).format[C]
)(JsonExample2.apply, unlift(JsonExample2.unapply))

That’s all folks!