Custom circe decoder for variant json-field - json

Question

How can I write circe decoder for class
case class KeyValueRow(count: Int, key: String)
where json contains field "count" (Int) and some extra string-field (name of this field may be various, like "url", "city", whatever)?
{"count":974989,"url":"http://google.com"}
{"count":1234,"city":"Rome"}

Answer 1

You can do what you need like this:
import io.circe.syntax._
import io.circe.parser._
import io.circe.generic.semiauto._
import io.circe.{ Decoder, Encoder, HCursor, Json, DecodingFailure}
object stuff{
case class KeyValueRow(count: Int, key: String)
implicit def jsonEncoder : Encoder[KeyValueRow] = deriveEncoder
implicit def jsonDecoder : Decoder[KeyValueRow] = Decoder.instance{ h =>
(for{
keys <- h.keys
key <- keys.dropWhile(_ == "count").headOption
} yield {
for{
count <- h.get[Int]("count")
keyValue <- h.get[String](key)
} yield KeyValueRow(count.toInt, keyValue)
}).getOrElse(Left(DecodingFailure("Not a valid KeyValueRow", List())))
}
}
import stuff._
val a = KeyValueRow(974989, "www.google.com")
println(a.asJson.spaces2)
val test1 = """{"count":974989,"url":"http://google.com"}"""
val test2 = """{"count":1234,"city":"Rome", "will be dropped": "who cares"}"""
val parsedTest1 = parse(test1).flatMap(_.as[KeyValueRow])
val parsedTest2 = parse(test2).flatMap(_.as[KeyValueRow])
println(parsedTest1)
println(parsedTest2)
println(parsedTest1.map(_.asJson.spaces2))
println(parsedTest2.map(_.asJson.spaces2))
Scalafiddle: link here
As I mentioned in the comment above, keep in mind that the that if you decode some json, and then re-encode it, the result will be different from the initial input. To fix that, you would need to keep track of the original name of the key field.