JSON decode nested field as Map[String, String] in Scala using circe - json

Question

A circe noob here. I am trying to decode a JSON string to case class in Scala using circe. I want one of the nested fields in the input JSON to be decoded as a Map[String, String] instead of creating a separate case class for it.
Sample code:
import io.circe.parser
import io.circe.generic.semiauto.deriveDecoder
case class Event(
action: String,
key: String,
attributes: Map[String, String],
session: String,
ts: Long
)
case class Parsed(
events: Seq[Event]
)
Decoder[Map[String, String]]
val jsonStr = """{
"events": [{
"ts": 1593474773,
"key": "abc",
"action": "hello",
"session": "def",
"attributes": {
"north_lat": -32.34375,
"south_lat": -33.75,
"west_long": -73.125,
"east_long": -70.3125
}
}]
}""".stripMargin
implicit val eventDecoder = deriveDecoder[Event]
implicit val payloadDecoder = deriveDecoder[Parsed]
val decodeResult = parser.decode[Parsed](jsonStr)
val res = decodeResult match {
case Right(staff) => staff
case Left(error) => error
}
I am ending up with a decoding error on attributes field as follows:
DecodingFailure(String, List(DownField(north_lat), DownField(attributes), DownArray, DownField(events)))
I found an interesting link here on how to decode JSON string to a map here: Convert Json to a Map[String, String]
But I'm having little luck as to how to go about it.
If someone can point me in the right direction or help me out on this that will be awesome.

Answer 1

Let's parse the error :
DecodingFailure(String, List(DownField(geotile_north_lat), DownField(attributes), DownArray, DownField(events)))
It means we should look in "events" for an array named "attributes", and in this a field named "geotile_north_lat". This final error is that this field couldn't be read as a String. And indeed, in the payload you provide, this field is not a String, it's a Double.
So your problem has nothing to do with Map decoding. Just use a Map[String, Double] and it should work.

Answer 2

So you can do something like this:
final case class Attribute(
key: String,
value: String
)
object Attribute {
implicit val attributesDecoder: Decoder[List[Attribute]] =
Decoder.instance { cursor =>
cursor
.value
.asObject
.toRight(
left = DecodingFailure(
message = "The attributes field was not an object",
ops = cursor.history
)
).map { obj =>
obj.toList.map {
case (key, value) =>
Attribute(key, value.toString)
}
}
}
}
final case class Event(
action: String,
key: String,
attributes: List[Attribute],
session: String,
ts: Long
)
object Event {
implicit val eventDecoder: Decoder[Event] = deriveDecoder
}
Which you can use like this:
val result = for {
json <- parser.parse(jsonStr).left.map(_.toString)
obj <- json.asObject.toRight(left = "The input json was not an object")
eventsRaw <- obj("events").toRight(left = "The input json did not have the events field")
events <- eventsRaw.as[List[Event]].left.map(_.toString)
} yield events
// result: Either[String, List[Event]] = Right(
// List(Event("hello", "abc", List(Attribute("north_lat", "-32.34375"), Attribute("south_lat", "-33.75"), Attribute("west_long", "-73.125"), Attribute("east_long", "-70.3125")), "def", 1593474773L))
// )
You can customize the Attribute class and its Decoder, so their values are Doubles or Jsons.