Parsing a simple array with Spray-json

Question

I'm trying (and failing) to get my head around how spray-json converts json feeds into objects. If I have a simple key -> value json feed then it seems to work ok but the data I want to read comes in a list like this:

[{
    "name": "John",
    "age": "30"
},
{
    "name": "Tom",
    "age": "25"
}]

And my code looks like this:

package jsontest

import spray.json._
import DefaultJsonProtocol._

object JsonFun {

  case class Person(name: String, age: String)
  case class FriendList(items: List[Person])

  object FriendsProtocol extends DefaultJsonProtocol {
    implicit val personFormat = jsonFormat2(Person)
    implicit val friendListFormat = jsonFormat1(FriendList)
  }

  def main(args: Array[String]): Unit = {

    import FriendsProtocol._

    val input = scala.io.Source.fromFile("test.json")("UTF-8").mkString.parseJson

    val friendList = input.convertTo[FriendList]

    println(friendList)
  }

}    

If I change my test file so it just has a single person not in an array and run val friendList = input.convertTo[Person] then it works and everything parses but as soon as I try and parse an array it fails with the error Object expected in field 'items'

Can anyone point me the direction of what I'm doing wrong?

Answer 1

Well, as is often the way immediately after posting something to StackOverflow after spending hours trying to get something working, I've managed to get this to work.

The correct implementation of FriendsProtocol was:

object FriendsProtocol extends DefaultJsonProtocol {
  implicit val personFormat = jsonFormat2(Person)
  implicit object friendListJsonFormat extends RootJsonFormat[FriendList] {
    def read(value: JsValue) = FriendList(value.convertTo[List[Person]])
    def write(f: FriendList) = ???
  } 
}

Telling Spray how to read / write (just read in my case) the list object is enough to get it working.

Hope that helps someone else!