JSON decode from object to string, and encode it back - json

Question

I am trying to decode a JSON string to case class and also transform one of the field from Object to JSON String.
Currently, I have only finished the decoding part, but don't know how to implement the encoding part.
Here is my code:
case class Foo(objStr: String)
implicit val encoder: Encoder[Foo] = deriveEncoder[Foo]
implicit val decoder: Decoder[Foo] = deriveDecoder[Foo].prepare { cursor =>
cursor.withFocus { json =>
json.mapObject { o =>
o.add("objStr", o("objStr").map(_.noSpaces.asJson).getOrElse(Json.Null))
}
}
}
val json =
"""{
| "objStr": {
| "0": 1,
| "1": 3.4,
| "2": "abc"
| }
|}""".stripMargin
val foo = io.circe.parser.decode[Foo](json).right.get
println(foo)
// output `{"0":1,"1":3.4,"2":"abc"}` as expected
println(foo.asJson.noSpaces)
// output `{"objStr":"{\"0\":1,\"1\":3.4,\"2\":\"abc\"}"}`
// but I expect `{"objStr":{"0":1,"1":3.4,"2":"abc"}}`
Please help me to finish the encoding part.
Thank you.
Updated:
Found a workable solution:
implicit val encoder: Encoder[Foo] = deriveEncoder[Foo].mapJson { json =>
json.mapObject { o =>
o.add("objStr", o("objStr").flatMap(_.asString).flatMap(parse(_).toOption).getOrElse(Json.Null))
}
}