How to generate json string if the `Json.obj` contains a `None` value?

Question

In playframework:

import play.api.libs.json._

val obj = Json.obj(
  "aaa" -> 111,
  "bbb" -> Some(222)
)

println(obj.toString)

Which outputs:

{"aaa":111,"bbb":222}

But if I change the code to:

val obj = Json.obj(
  "aaa" -> 111,
  "bbb" -> None
)

It can't be compiled, and reports:

Error:(6, 17) diverging implicit expansion for type play.api.libs.json.Writes[None.type]
starting with method OptionWrites in trait DefaultWrites
  "bbb" -> None
           ^

How to fix it?

It should be `None`, the value is actually from a field of other objects, which is an `Option` — Freewind, Apr 15 '14 at 06:57
If it is `None`. The corresponding json should not contains this field, rather than return a javascript `null` value. — jilen, Apr 15 '14 at 06:59
How about define a Writes[XXX]？ Using [macro](http://www.playframework.com/documentation/2.2.x/ScalaJsonInception) — jilen, Apr 15 '14 at 07:04

score 0 · Answer 1 · answered Apr 15 '14 at 07:49

Json.obj works fine with None if it is a field of an object as you say:

case class A(aaa: Int, bbb: Option[Int])
defined class A

val a = A(1, None)
a: A = A(1,None)

Json.obj("aaa" -> a.aaa, "bbb" -> a.bbb)
res1: play.api.libs.json.JsObject = {"aaa":1,"bbb":null}

You can make your example work as well if your None is an Option, not just None.type as compiler says:

Json.obj("aaa" -> 111, "bbb" -> None.asInstanceOf[Option[Int]])
res3: play.api.libs.json.JsObject = {"aaa":111,"bbb":null}

Json.obj("aaa" -> 111, "bbb" -> Option[String](null))
res4: play.api.libs.json.JsObject = {"aaa":111,"bbb":null}

How to generate json string if the `Json.obj` contains a `None` value?

1 Answers1