Explicitly output JSON null in case of missing optional value

Question

Consider this example using Play's JSON API (play.api.libs.json):

case class FooJson(
   // lots of other fields omitted
   location: Option[LocationJson]
)

object FooJson {
  implicit val writes = Json.writes[FooJson]
}

and

case class LocationJson(latitude: Double, longitude: Double)

object LocationJson {
  implicit val writes = Json.writes[LocationJson]
}

If location is None, the resulting JSON won't have location field at all. This is fine and understadable. But if I wanted for some reason (say, to make my API more self-documenting), how can I explicitly output null in JSON?

{      
  "location": null
}

I also tried defining the field as location: LocationJson and passing option.orNull to it, but it does not work (scala.MatchError: null at play.api.libs.json.OWrites$$anon$2.writes). For non-custom types such as String or Double, this approach would produce null in JSON output.

So, while using Json.writes[FooJson] as shown above (or something equally simple, i.e. not having to write a custom Writes implementation), is there a clean way to include nulls in JSON?

What I'm asking is analogous to JsonInclude.Include.ALWAYS in the Jackson library (also Jackson's default behaviour). Similarly in Gson this would be trivial
(new GsonBuilder().serializeNulls().create()).

Play 2.4.4

Answer 1

Greg Methvin, a Play committer, wrote this answer to me in a related GitHub issue:

The JSON macros only support one way of encoding optional values, which is to omit None values from the JSON. This is not a bug but rather a limitation of the implementation. If you want to include nulls you're unfortunately going to have to implement your own Writes.

I do think we should try to provide more configurability for the macros though.

In this case, I'll let Play exclude this field when the value is null, even if it slightly sacrifices API consistency and self-documentability. It's still such a minor thing (in this particular API) that it doesn't warrant uglifying the code as much as a custom Writes would take for a case class with a dozen values.

I'm hoping they do make this more configurable in future Play versions.