I have a problem with adding a field to Json object in Play Framework using Scala:
I have a case class containing data. For example:
case class ClassA(a:Int,b:Int)
and I am able to create a Json object using Json Writes:
val classAObject = ClassA(1,2)
implicit val classAWrites= Json.writes[ClassA]
val jsonObject = Json.toJson(classAObject)
and the Json would look like:
{ a:1, b:2 }
Let's suppose I would like to add an additional 'c' field to the Json object. Result:
{ a:1, b:2, c:3 }
How do I do that without creating a new case class or creating my Json object myself using Json.obj? I am looking for something like:
jsonObject.merge({c:3})
Any help appreciated!
JsObject
has a +
method that allows you to add fields to an object, but unfortunately your jsonObject
is statically typed as a JsValue
, not a JsObject
. You can get around this in a couple of ways. The first is to use as
:
scala> jsonObject.as[JsObject] + ("c" -> Json.toJson(3))
res0: play.api.libs.json.JsObject = {"a":1,"b":2,"c":3}
With as
you're essentially downcasting—you're telling the compiler, "you only know that this is a JsValue
, but believe me, it's also a JsObject
". This is safe in this case, but it's not a good idea. A more principled approach is to use the OWrites
directly:
scala> val jsonObject = classAWrites.writes(classAObject)
jsonObject: play.api.libs.json.JsObject = {"a":1,"b":2}
scala> jsonObject + ("c" -> Json.toJson(3))
res1: play.api.libs.json.JsObject = {"a":1,"b":2,"c":3}
Maybe someday the Json
object will have a toJsonObject
method that will require a OWrites
instance and this overly explicit approach won't be necessary.