Polymorphic serializer, ignore unregistered objects when deserializing list of polymorphic objects

I have made a serializer class for the json:api specificaiton.

@Serializable
data class JsonApiResponseItem(
        override val jsonapi: JsonApiVersion,
        override val included: List<@Polymorphic JsonApiObject>? = null,
        @Polymorphic val data: JsonApiObject): JsonApiResponse() {

}

included can return many different types of objects which I am successfully using the polymorphic feature to deserialize. But this only works if all possible object types are registered in the scope of class JsonApiObject for polymorphic serialization.

If any types are not registered an error like is thrown

io.ktor.client.call.ReceivePipelineException: Fail to run receive pipeline: 
kotlinx.serialization.SerializationException: example-type is not registered for polymorphic 
serialization in the scope of class api.jsonapi.JsonApiObject

Is it possible to ignore deserializing polymorphic type objects which are not registered for polymorphic serialization without throwing an error? This would really be helpful because it would prevent deserialization from throwing errors if any new objects added to the included list… and it would be convenient if don’t actually need to deserialize some of the included objects

[luca992]

Yeah, that sounds right. Maybe another option would be to allow this in nonstrict mode by allowing a backup serializer to be registered for polymorphic classes in SerializersModule.

maybe something along the lines of:

@Serializable
data class EmptyJsonApiObject : JsonApiObject()

Json(context = SerializersModule {
    polymorphic(JsonApiObject::class) {
        JsonApiObjectUser::class with JsonApiObjectUser.serializer()
        JsonApiObjectEmployee::class with JsonApiObjectEmployee.serializer()
        addBackUpSerializer(EmptyJsonApiObject.serializer())
    }
},
        configuration = JsonConfiguration(
                strictMode = false,
        )
)

//addBackUpSerializer would require EmptyJsonApiObject to extend JsonApiObject

So in nonStrict mode at least, instead of throwing kotlinx.serialization.SerializationException: example-type is not registered for polymorphic serialization in the scope of class api.jsonapi.JsonApiObject, it would attempt to de-serialize using EmptyJsonApiObject’s deserializer. Which I think should work fine, because nonStrict allows deserializing to an empty class.

My suggestion sounds pretty hacky though. But might be something to consider until support for skipping malformed element can be added.

[ge-org]

I know this issue is pretty old by now, however, I just stumbled upon it since I had the same problem. There’s another solution available that doesn’t require a wrapper or custom serializer.

We can register a default serializer for polymorphic types that will be used as a fallback. The downside compared to the wrapper solution is that we must register the serializer for every new polymorphic type we add. So it requires some discipline.

val json = Json {
    serializersModule += SerializersModule {
        polymorphic(MyPolymorphicType::class) {
            defaultDeserializer { MyPolymorphicType.Unknown.serializer() }
        }
    }
}

https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#default-polymorphic-type-handler-for-deserialization