Play 2.x Json transform json keys to camelCase from underscore case

I want to transform a json with underscore case keys to camel case keys.

"{\"first_key\": \"first_value\", \"second_key\": {\"second_first_key\":\"second_first_value\"}}"

to

 "{\"firstKey\": \"first_value\", \"secondKey\": {\"secondFirstKey\":\"second_first_value\"}}"

This is partial code:

val CamelCaseRegex = new Regex("(_.)")
val jsonTransformer = (__).json.update(
  //converts json camel_case field names to Scala camelCase field names  
)
val jsonRet = Json.parse(jsonStr).transform(jsonTransformer)

I have tried several ways in the update method without success.

While it would be nice to do this with just the native Play library, it's a good use-case for Mandubian's Play Json Zipper extension libraries.

Here's a quick go at this (not exhaustively tested). First you need to add the resolver and library to your build:

resolvers += "mandubian maven bintray" at "http://dl.bintray.com/mandubian/maven"

libraryDependencies ++= Seq(
  "com.mandubian"     %% "play-json-zipper"    % "1.2"
)

Then you could try something like this:

import play.api.libs.json._
import play.api.libs.json.extensions._

// conversion function borrowed from here:
// https://gist.github.com/sidharthkuruvila/3154845
def underscoreToCamel(name: String) = "_([a-z\\d])".r.replaceAllIn(name, {m =>
  m.group(1).toUpperCase
})

// Update the key, otherwise ignore them...
// FIXME: The None case shouldn't happen here so maybe we
// don't need it...
def underscoreToCamelCaseJs(json: JsValue) = json.updateAllKeyNodes {
  case (path, js) => JsPathExtension.hasKey(path) match {
    case Some(key) => underscoreToCamel(key) -> js
    case None => path.toJsonString -> js
  }
}

Which on this input:

val testJson = Json.obj(
  "some_str" -> JsString("foo_bar"),
  "some_obj" -> Json.obj(
    "some_field" -> Json.arr("foo", "bar")
  ),
  "an_int" -> JsNumber(1)
)

...produces:

{
  "someStr" : "foo_bar",
  "someObj" : {
    "someField" : [ "foo", "bar" ]
  },
  "anInt" : 1
}