Circe Encoding Sealed Traits and Co Product Case class Fails - json

Question

I have the following case class:
case class SmartMeterData(
dateInterval: SmartMeterDataInterval = HalfHourInterval(),
powerUnit: PowerUnit = KWH,
smartMeterId: String,
timestamp: String,
value: Double
) {
def timeIntervalInDuration: FiniteDuration = dateInterval match {
case HalfHourInterval(_) => FiniteDuration(30, TimeUnit.MINUTES)
case FullHourInterval(_) => FiniteDuration(60, TimeUnit.MINUTES)
}
}
object SmartMeterData {
sealed trait SmartMeterDataInterval { def interval: String }
case class HalfHourInterval(interval: String = "HH") extends SmartMeterDataInterval
case class FullHourInterval(interval: String = "FH") extends SmartMeterDataInterval
sealed trait PowerUnit
case object WH extends PowerUnit
case object KWH extends PowerUnit
case object MWH extends PowerUnit
}
I just wrote a very simple unit test to see if Circe works for my scenario:
"SmartMeterData" should "successfully parse from a valid JSON AST" in {
val js: String = """
{
"dateInterval" : "HH",
"powerUnit" : "KWH",
"smartMeterId" : "LCID-001-X-54",
"timestamp" : "2012-10-12 00:30:00.0000000",
"value" : 23.0
}
"""
val expectedSmartMeterData = SmartMeterData(smartMeterId = "LCID-001-X-54", timestamp = "2012-10-12 00:30:00.0000000", value = 23.0)
decode[SmartMeterData](js) match {
case Left(err) => fail(s"Error when parsing valid JSON ${err.toString}")
case Right(actualSmartMeterData) =>
assert(actualSmartMeterData equals expectedSmartMeterData)
}
}
But it fails with the following error:
Error when parsing valid JSON DecodingFailure(CNil, List(DownField(dateInterval)))
Is there a known limitation with circe where it does not yet work for my case above?

Answer 1

There's no reason that Circe wouldn't work, but the way you've designed the data model, there's basically zero chance of any Scala JSON library that can automatically generate an encoder/decoder working without a lot of manual work.
For example
sealed trait SmartMeterDataInterval { def interval: String }
case class HalfHourInterval(interval: String = "HH") extends SmartMeterDataInterval
case class FullHourInterval(interval: String = "FH") extends SmartMeterDataInterval
the schema for both, in any Scala JSON library which automatically derives instances for case classes, would be along the lines of
{ "interval": "HH" }
for HalfHourInterval and
{ "interval": "FH" }
for FullHourInterval (i.e. because each has a single string field named interval, they're effectively the same class). In fact your model allows you to have FullHourInterval("HH"), which for at least one method of generating a decoder for an ADT hierarchy in circe (the one in the documentation which uses shapeless) would be the result of decoding { "interval": "HH" }, since that essentially takes the first constructor in lexical order which matches (i.e. FullHourInterval). If the intent is to only allow full- or half-hour intervals, then I'd suggest expressing that as:
case object HalfHourInterval extends SmartMeterDataInterval { def interval: String = "HH" }
case object FullHourInterval extends SmartMeterDataInterval { def interval: String = "FH" }
I'm not directly familiar with how circe encodes case objects, but you can pretty easily define an encoder and decoder for SmartMeterDataInterval:
object SmartMeterDataInterval {
implicit val encoder: Encoder[SmartMeterDataInterval] =
Encoder.encodeString.contramap[SmartMeterDataInterval](_.interval)
implicit val decoder: Decoder[SmartMeterDataInterval] =
Decoder.decodeString.emap {
case "HH" => Right(HalfHourInterval)
case "FH" => Right(FullHourInterval)
case _ => Left("not a valid SmartMeterDataInterval")
}
}
You would then do something similar to define an Encoder/Decoder for PowerUnit