How to combine polymorphic and transforming serializers for nested JSON, using Kotlin serialization? - kotlin

Question

This is a simplified example of the JSON I want to work with:
{
"useless_info": "useless info",
"data": {
"useless_info2": "useless info 2",
"children": [
{
"kind": "Car",
"data": {
"id": 1,
"transmission": "manual"
}
},
{
"kind": "Boat",
"data": {
"id": 2,
"isDocked": true
}
}
]
}
}
children is an array of vehicle objects. vehicle can be boat or car.
My Problem
The information that I want is nested quite deep (the real JSON is much deeply nested). A hack solution is to model the JSON exactly by writing dozens of nested data classes that references each other. I do not want to do this.
My problem is that while I know how to use JsonTransformingSerializer to unwrap arrays of a single type, and JsonContentPolymorphicSerializer to work with objects of various types, in this situation I believe I require both, but I can't get it to work.
What I Did
Assuming a single type
I tried understanding how it would work if it was a single type.
If the objects I wanted where all of the same type, it would be trivial to implement a JsonTransformingSerializer to cut right into the data I want. In this example, I will assume I only care about the ID, so I can just create a generic Vehicle model.
#Serializable
data class VehicleResponse(
#Serializable(with = VehicleResponseSerializer::class)
#SerialName("data")
val vehicles: List<Vehicle>
)
#Serializable
data class Vehicle(val id: Int)
object VehicleResponseSerializer : JsonTransformingSerializer<List<Vehicle>>(ListSerializer(Vehicle.serializer())) {
override fun transformDeserialize(element: JsonElement): JsonElement {
val vehicles = mutableListOf<JsonElement>()
// equals: [{"kind":"Car","data":{"id":1,"transmission":"manual"}},{"kind":"Boat","data":{"id":2,"isDocked":true}}]
val vehicleArray = element.jsonObject["children"]!!.jsonArray
vehicleArray.forEach { vehicle ->
// equals: {"id":1,"transmission":"manual"}}
val vehicleData = vehicle.jsonObject["data"]!!
vehicles.add(vehicleData)
}
return JsonArray(vehicles.toList())
}
}
The code works perfectly. Calling it out from main, printing the result gives me:
VehicleResponse(vehicles=[Vehicle(id=1), Vehicle(id=2)])
But they are actually Polymorphic!
Assuming one type does not work. I need to work with Car and Boat, and call their respective functions and properties.
I tried to model the structure like this:
#Serializable
data class VehicleResponse(
#Serializable(with = VehicleResponseSerializer::class)
#SerialName("data")
val vehicles: List<Vehicle>
)
#Serializable
abstract class Vehicle {
abstract val id: Int
}
#Serializable
data class Car(
override val id: Int,
val transmission: String,
) : Vehicle()
#Serializable
data class Boat(
override val id: Int,
val isDocked: Boolean,
) : Vehicle()
What I Want
I want to receive a JSON from a server, and instantly be able to deserialize it into a list of Vehicle objects, like the one VehicleResponse has.
I want to navigate through a deeply nested JSON, and unwrap an array that contains various Vehicle objects. For this, I assume I need to use JsonTransformingSerializer.
I want to use polymorphic deserialization to convert each of Vehicle into its corresponding subtype.
The actual ACTUAL problem
The thing that is truly throwing me in a loop is that a polymorphic serializer just does not seem to fit. It's called first before I get to parse the JSON. How am I supposed to decide which serializer to use?
Here's a test implementation:
object VehiclePolymorphicSerializer: JsonContentPolymorphicSerializer<VehicleResponse>(VehicleResponse::class) {
override fun selectDeserializer(element: JsonElement): DeserializationStrategy<out VehicleResponse> {
println("\nselectDeserializer()\n" +
"base element:\n" +
"$element\n")
// this return is a temporary hack, I just want to see the base element by printing it to the console
return VehicleResponse.serializer()
}
}
It prints:
selectDeserializer()
base element:
{"useless_info":"useless info","data":{"useless_info2":"useless info 2","children":[{"kind":"Car","data":{"id":1,"transmission":"manual"}},{"kind":"Boat","data":{"id":2,"isDocked":true}}]}}
That's the whole initial JSON! How am I supposed to decide which deserialization strategy to use, if both Car and Boat are in there? The JsonTransformingSerializer is called after the JsonContentPolymorphicSerializer.
Really not sure how am I supposed to proceed here. Would really appreciate even a slight hint.

Answer 1

kotlinx.serialization can handle polymorphic deserialization in this case without custom JsonContentPolymorphicSerializer. You just need to preserve class descriminator in the JSON returned from your JsonTransformingSerializer:
object VehicleResponseSerializer : JsonTransformingSerializer<List<Vehicle>>(ListSerializer(Vehicle.serializer())) {
override fun transformDeserialize(element: JsonElement): JsonElement {
// equals: [{"kind":"Car","data":{"id":1,"totalWheels":"4"}},{"kind":"Boat","data":{"id":2,"isDocked":true}}]
val vehicleArray = element.jsonObject["children"]!!.jsonArray
// equals: [{"type":"Car","id":1,"totalWheels":"4"}, {"type":"Boat","id":2,"isDocked":true}]
return JsonArray(vehicleArray.map {
val data = it.jsonObject["data"]!!.jsonObject
val type = it.jsonObject["kind"]!!
JsonObject(
data.toMutableMap().apply { this["type"] = type }
/*
//Kotlin 1.4 offers a nicer way to do this:
buildMap {
putAll(data)
put("type", type)
}
*/
)
})
}
}
If you declare Vehicle class as sealed (not just abstract), you're already good to go. If you want to keep it abstract, then you need to register all its subclasses in serializersModule
val module = SerializersModule {
polymorphic(Vehicle::class) {
subclass(Car::class)
subclass(Boat::class)
}
}
and pass it to JSON configuration
val kotlinx = Json {
ignoreUnknownKeys = true
serializersModule = module
}
UPDATE
(Alternative approach with combination of JsonTransformingSerializer & JsonContentPolymorphicSerializer)
Actually, it's possible to combine these two serializers.
For the sake of justification for this approach, let's imagine that original JSON doesn't have that nice kind field, and we have to figure out the actual subtype of Vehicle by the shape of JSON. In this case it could be the following heuristic: "if there is a isDocked field, then it's a Boat, othrewise - a Car".
Yes, we may include this logic into JsonTransformingSerializer to create class descriminator on the fly:
val type = when {
"isDocked" in data -> JsonPrimitive("Boat")
else -> JsonPrimitive("Car")
}
But it's more common (and type-safe) to use JsonContentPolymorphicSerializer for this. So we may simplify JsonTransformingSerializer:
object VehicleResponseSerializer : JsonTransformingSerializer<List<Vehicle>>(ListSerializer(Vehicle.serializer())) {
override fun transformDeserialize(element: JsonElement): JsonElement {
// equals: [{"kind":"Car","data":{"id":1,"totalWheels":"4"}},{"kind":"Boat","data":{"id":2,"isDocked":true}}]
val vehicleArray = element.jsonObject["children"]!!.jsonArray
// equals: [{"id":1,"totalWheels":"4"}, {"id":2,"isDocked":true}] // Note that class discriminator is absent here!
return JsonArray(vehicleArray.map { it.jsonObject["data"]!! })
}
}
and define JsonContentPolymorphicSerializer (for Vehicle, not for VehicleResponse!) to handle actual serializer selection:
object VehiclePolymorphicSerializer : JsonContentPolymorphicSerializer<Vehicle>(Vehicle::class) {
override fun selectDeserializer(element: JsonElement): DeserializationStrategy<out Vehicle> = when {
"isDocked" in element.jsonObject -> Boat.serializer()
else -> Car.serializer()
}
}
Since JsonTransformingSerializer (aka VehicleResponseSerializer) is registered for vehicles field serialization it's called before JsonContentPolymorphicSerializer (aka VehiclePolymorphicSerializer). Actually, the latter one is not yet called at all. We need to explicitly register it in serializersModule and pass it to JSON configuration (regardless of whether Vehicle class is declared as abstract or sealed):
val module = SerializersModule {
polymorphicDefault(Vehicle::class) { VehiclePolymorphicSerializer }
}
val kotlinx = Json {
ignoreUnknownKeys = true
serializersModule = module
}