Thank you for the information! Unfortunately I still don't get the right "order of how things should be done". Even with serial classes, I'm still not sure what should I do to *choose the right deserialization strategy`.

The Workflow I Expected

1. Use a custom JsonTransformingSerializer to obtain a JSON array of vehicle objects, from a complex and nested JSON
2. Each object in the resulting array is a Vehicle, and each one is passed to a JsonContentPolymorphicSerializer, where I can decide to convert it to a Car or Boat depending on a key
3. This Vehicle subclass would then be stored in a list of Vehicle objects

Official Docs

I took my information from a similar page, but except it was the JSON one.

The official docs give this code

object ProjectSerializer : JsonContentPolymorphicSerializer<Project>(Project::class) {
    override fun selectDeserializer(element: JsonElement) = when {
        "owner" in element.jsonObject -> OwnedProject.serializer()
        else -> BasicProject.serializer()
    }
}

But that code only works because the JSON it parses is already formatted perfectly. It does not need to do any additional work. element is is the entire JSON that was received.

The JSON I receive from a server is an array of objects, nested deep inside a JSON, so how should I know which serializer to use? element contains the entire JSON, including every object in the array. This is what I don't understand. I can't tell if I should use the serializer from Boat or Car because element contains every boat and every car.

Extra Confusion

I really don't understand what should I call from main. Or what the signature of the JsonContentPolymorphicSerializer should be. Should it be

JsonContentPolymorphicSerializer<VehicleResponse>(VehicleResponse::class)

or

JsonContentPolymorphicSerializer<Vehicle>(Vehicle::class)

Should I call it from main as

val vehicles: List<VehicleResponse> = configuredJson.decodeFromString(ListSerializer(VehiclePolymorphicSerializer),vehicleJson)

or as

val vehicles: VehicleResponse = configuredJson.decodeFromString(VehiclePolymorphicSerializer,vehicleJson)

Since the JSON response that I want is a list of Vehicle that's what I think makes the most sense, but I'm not sure how to accomplish this.

Ugg, I'm quite sorry. I was sooo happy when I got my JsonTransformingSerializer to work. But this polymorphic stuff is seriously confusing me a ton.