Circe: force an optional field to `null`

Question

Is there a way to serialize a single None field to "null" ?

For example:

// When None, I'd like to serialize only f2 to `null`
case class Example(f1: Option[Int], f2: Option[Int])
val printer = Printer.noSpaces.copy(dropNullValues = true)

Example(None, None).asJson.pretty(printer) === """{"f2":null}"""

Answer 1

You can do this pretty straightforwardly by mapping a filter over the output of an encoder (which can be derived, defined with Encoder.forProductN, etc.):

import io.circe.{ Json, ObjectEncoder }
import io.circe.generic.semiauto.deriveEncoder

case class Example(f1: Option[Int], f2: Option[Int])

val keepSomeNulls: ((String, Json)) => Boolean = {
  case ("f1", v) => !v.isNull
  case (_, _) => true
}

implicit val encodeExample: ObjectEncoder[Example] =
  deriveEncoder[Example].mapJsonObject(_.filter(keepSomeNulls))

And then:

scala> import io.circe.syntax._
import io.circe.syntax._

scala> Example(Some(1), Some(2)).asJson.noSpaces
res0: String = {"f1":1,"f2":2}

scala> Example(Some(1), None).asJson.noSpaces
res1: String = {"f1":1,"f2":null}

scala> Example(None, Some(2)).asJson.noSpaces
res2: String = {"f2":2}

scala> Example(None, None).asJson.noSpaces
res3: String = {"f2":null}

Note that configuring a printer to drop null values will still remove "f2": null here. This is part of the reason I think in general it's best to make the preservation of null values solely the responsibility of the printer, but in a case like this, the presence or absence of null-valued fields is clearly semantically meaningful, so you kind of have to mix up the levels.