Serializing a Scala List to JSON in Play2

Question

I am trying to deserialize a list of Scala objects to a JSON map in Play2 - a pretty trivial use case with JSON, I'd say. My JSON output would be something along the lines of:

{
    "users": [
        {
            "name": "Example 1",
            "age": 20
        },
        {
            "name": "Example 2",
            "age": 42
        }
    ]
}

To achieve this I am looking at the Play2's JSON documentation titled "The Play JSON library". To me their examples are pretty trivial, and I've confirmed that they work for me. Hence, I am able to deserialize a single User object properly.

But making a map containing a list in JSON seems a bit verbose in Play2, when I read the documentation. Is there something I am not grokking?

This is basically my simple Scala code:

case class User(name: String, age: Int)

object UserList {
  implicit val userFormat = Json.format[User]  

  val userList = List(User("Example 1", 20), User("Example 2", 42))
  val oneUser = Json.toJson(userList(0)) // Deserialize one Scala object properly to JSON.
  // JSON: { "user" : [ <-- put content of userList here. How?
  //                  ]
  //       }
}

So my question would be; how can I transform the content of the userList List above to a hash in the JSON in a more generic way than explicitly writing out each hash element, as the Play documentation suggests?

Solution

scala> import play.api.libs.json._
import play.api.libs.json._

scala> case class User(name: String, age: Int)
defined class User

scala> implicit val userFormat = Json.format[User]
userFormat: play.api.libs.json.OFormat[User] = play.api.libs.json.OFormat$$anon$1@38d2c662

scala> val userList = List(User("Example 1", 20), User("Example 2", 42))
userList: List[User] = List(User(Example 1,20), User(Example 2,42))

scala> val users = Json.obj("users" -> userList)
users: play.api.libs.json.JsObject = {"users":[{"name":"Example 1","age":20},{"name":"Example 2","age":42}]}

Comments

[Johan Paul]

Oh wow - that was simple! I have to read the docs again. Wonder why this hasn't been told more clearly - or I missed it. Thanks!!

[Johan Paul]

I must say, that just by reading the docs at www.playframework.com about Play's JSON support, I would have never came up with how to achieve what I wanted - and what you pasted. The docs seem to be quite rubbish.

[user5880801]

@JohanPaul yeah, at some point you have to look at the method signatures and read the source code (to see what instances of the Writes typeclass are already defined by Play, in this particular case).

[angelokh]

@IonuțG.Stan How did you import all play jars to scala console? I tried to reproduce your commands but I can't.

[user5880801]

@angelokh I started the scala console from inside an sbt session in a Play project directory I had locally. I've described an alternative in this blog post igstan.ro/posts/…. Hope it helps.

[Ilan Biala]

Can someone explain what each line does? I'm fairly new to Scala so I'm not sure how some of the code is working.

[Zee]

Way better than the play documentation. They should pay you.

[Kevin Meredith]

For this example alone, it's sufficient to use implicit val userReads = Json.writes[User] rather than Json.format....