Is it possible to do polymorphic serialization based on the presence or absence of a field? I want t...

# serialization

[kevin.cianfarini]

Is it possible to do polymorphic serialization based on the presence or absence of a field? I want to have the following hierarchy for a DB entity

sealed interface Entity {
  val commonProperty: Int

  class New(override val commonProperty: Int) : Entity
  class Persisted(
    val id: Long,
    override val commonProperty: Int
  ) : Entity
}

Given the following json

[
  { "commonProperty": 0 },
  { "id": 1, "commonProperty": 0 }
]

I would like that to deserialize to the following in Kotlin

[New(commonProperty = 0), Persisted(id = 1, commonProperty = 0)]

[kevin.cianfarini]

I realize a more simple model could look like the following:

class Entity(val id: Long?, val commonProperty: Int)

but that would require either passing in null explcitily in JSON, or disabling Json.explicitNulls globally which I'd like to avoid.

[zt]

You could set a default parameter value for id
Kotlinx serialization would then deserialize to that value if it's missing in the input

[Adam S]

yes, you can create a JsonContentPolymorphicSerializer that can check if a JSON property is present or missing, and then select the serializer you’d like to use

https://github.com/Kotlin/kotlinx.serialization/blob/v1.4.1/docs/json.md#content-based-polymorphic-deserialization

[kevin.cianfarini]

@Adam S I don't know how I didn't see this in the docs! This is fantastic. Thank you.