I have a class structure roughly like: ```sealed class MessagePayload ... data class PayloadOne(val...

# serialization

[bbaldino]

I have a class structure roughly like:

sealed class MessagePayload ...

data class PayloadOne(val foo: String) : MessagePayload ...

class Message(val msgType: MessageType, val payload: MessagePayload) { ... }

and I was hoping to get it to serialize as:

{
  "msgType": "payloadOne",
  "payload": {
    "foo": "..."
  }
}

that is: have the discriminant for the 'payload' field actually be at a layer "above", is that possible via just annotations? or would it require custom serializer/deserializer?

[Dominaezzz]

That json payload is possible with annotations but you'll have to change your class structure. Otherwise you have to write your own serializer.

[bbaldino]

oh, that'd be great if i could achieve it with annotations...i've been looking around at custom serializers and not feeling too encouraged. how would the class structure have to change?

[Dominaezzz]

sealed class MessagePayload ...

data class PayloadOne(val foo: String) : MessagePayload ...

sealed class Message {
  abstract val msgType: MessageType
  abstract val payload: MessagePayload
}

data class PayloadOneMessage(override val msgType: MessageType, override val payload: PayloadOne) : Message()

[bbaldino]

ah, and then explicitly serialize as PayloadOneMessage and not Message, right?

[Dominaezzz]

Ehh, well if you want the discriminator then you have to use Message .

[bbaldino]

ok, i'll play around with that a bit. thanks!

[bbaldino]

this worked great, thank you!

[bbaldino]

i'm a bit confused, actually, why this doesn't serialize the msgType field? i end up with something like:

{
  "type": "MyMsgType",
  "payload": {
    "foo": "hello"
  }
}

which is what i want, but why doesn't msgType get serialized there?

[Dominaezzz]

Not sure. I'd have to see code.

[bbaldino]

(trying to put a playground link together, but from what i can tell they don't have kotlinx serialization available there)

[bbaldino]

here's a playground with the code, even though it won't run there: https://pl.kotl.in/edW2zL-NX

[bbaldino]

hm, the msgType field is omitted from the tostring as well, for example:

println(PayloadOneMessage(PayloadOne("hello")))

prints PayloadOneMessage(payload=PayloadOne(foo=hello))

[Dominaezzz]

It's because you removed it from the ctor.

[bbaldino]

ahhhh, because it's a data class

[Dominaezzz]

Try adding a @Required annotation to the field.

[bbaldino]

yeah, that did it. for my case it works out because i don't actually want it, but wanted to understand why it wasn't there.