How to implement implicit Json Writes of embedded object in Play Framework 2.x

It's a question of visibility, when declaring the implicit Writes[Foo] you are not making visible the implicit Writes[Bar] to it:

scala> :paste// Entering paste mode (ctrl-D to finish)import play.api.libs.json._case class Bar(x: String, y: Int)object Bar {  implicit val implicitBarWrites = new Writes[Bar] {    def writes(bar: Bar): JsValue = {      Json.obj(        "x" -> bar.x,        "y" -> bar.y      )    }  }}case class Foo(id: String, bar: Bar)object Foo {  import Bar._  implicit val implicitFooWrites = new Writes[Foo] {    def writes(foo: Foo): JsValue = {      Json.obj(        "id" -> foo.id,        "bar" -> foo.bar      )     }   }     }// Exiting paste mode, now interpreting.import play.api.libs.json._defined class Bardefined module Bardefined class Foodefined module Fooscala> Json.prettyPrint(Json.toJson(Foo("23", Bar("x", 1))))res0: String = {  "id" : "23",  "bar" : {    "x" : "x",    "y" : 1  }}

Also, if you're using Play 2.1+ make sure to check out the brand new use of 2.10's macros: http://www.playframework.com/documentation/2.1.0/ScalaJsonInception

If you're happy with the use of the case classes and the val/vars' names being used as keys in the json output, as in your case BTW, then you can use the two one-liners:

implicit val barFormat = Json.writes[Bar]implicit val fooFormat = Json.writes[Foo]

That will give you the exact equivalent:

scala> import play.api.libs.json._import play.api.libs.json._scala> case class Bar(x: String, y: Int)defined class Barscala> case class Foo(id: String, bar: Bar)defined class Fooscala> implicit val barWrites = Json.writes[Bar]barWrites: play.api.libs.json.OWrites[Bar] = play.api.libs.json.OWrites$$anon$2@257cae95scala> implicit val fooWrites = Json.writes[Foo]fooWrites: play.api.libs.json.OWrites[Foo] = play.api.libs.json.OWrites$$anon$2@48f97e2ascala> Json.prettyPrint(Json.toJson(Foo("23", Bar("x", 1))))res0: String = {  "id" : "23",  "bar" : {    "x" : "x",    "y" : 1  }}