Kotlin JSON deserialization with nested class discriminator - json

Question

I want to use a class discriminator in Kotlin to get polymorphic deserialization of JSON messages. The problem is that the class discriminator in my messages is nested one level deep:
{
"header": {
"message_type": "FooBar"
}
"data": {...}
}
The contents of data depend on message_type. If message_type were one level higher, this would be trivial by setting #JsonClassDiscriminator("message_type") on the base class and #SerialName("FooBar") on the subclass. But since it is nested it is unclear how to solve this without introducing a ton of boilerplate serialization code.

Answer 1

Not the prettiest solution, but you can hack around this as such:
write your #Serializable object as if the discriminator is at the top level
when parsing, first use decodeFromString<JsonElement> to get a generic JSON object
use that object to move the discriminator to top-level
use decodeFromJsonElement to parse that
What step 3 looks like
// if json object has a message type in its header,
// return an object with that at top level so we can use it as a class discriminator
fun hoistDiscriminator(json: JsonElement): JsonElement {
if (json is JsonObject) {
val header = json["header"]
if (header is JsonObject) {
val discriminator = header["message_type"]
if (discriminator is JsonPrimitive && discriminator.isString) {
val map = json.toMutableMap()
map["message_type"] = discriminator
return JsonObject(map)
}
}
}
return json
}
and then the full parsing looks like this
json.decodeFromJsonElement(hoistDiscriminator(json.decodeFromString(response)))
There is a corner case in step 3: what if message_type is already defined at top-level? Handling that is left as an exercise to the reader.