I am facing an issue where the discriminator is a ...

# serialization

[gts13]

I am facing an issue where the discriminator is a name/value pair in the json file and my data classes fail on the serilization.

So I receive the following json file of a user:

{
	"header" : {...},
	"sources:": [
		{
			"discriminator" : "facebook",
			"facebook" : {
				"common" : {
	         		"lastUsedOn" : "2023-01-01T00:00:00Z"
	         		"device" : "computer"
				},
				"friends" : [...]
			}
		},
		{
			"discriminator" : "facebook",
			"facebook" : {
				"common" : {
	         		"lastUsedOn" : "2022-01-01T00:00:00Z"
	         		"device" : "mobile"
				},
				"friends" : [...]
			}
		},
		{
			"discriminator" : "instagram",
			"instagram" : {
				"common" : {
	         		"lastUsedOn" : "2000-01-01T00:00:00Z"
	         		"device" : "tablet"
				},
				"followers" : [...]
			}
		},
		{
			"discriminator" : "twitter",
			"twitter" : {
				"common" : {
	         		"lastUsedOn" : "2000-01-01T00:00:00Z"
	         		"device" : "mobile"
				},
				"tweets" : [...]
			}
		}
	]
}

and my data classes are the following:

Serializable
data class User(
    @SerialName("header")
    private val header: Header,

    @SerialName("sources")
    private val Sources: List<Source>,
)


@Serializable
sealed class Source {
    @SerialName("common")
    abstract val common: Common

    @Serializable
    data class Common(
    	@SerialName("lastUsedOn")
        var lastUsedOn: Instant,
        @SerialName("device")
        var device: String
    )
}


@Serializable
@SerialName("facebook")
data class Facebook(
	override val common: Common,
	@SerialName("friends")
	val friends: ...
) : Source()


@Serializable
@SerialName("instagram")
data class Instagram(
	override val common: Common,
	@SerialName("followers")
	val followers: List<...>
) : Source()


@Serializable
@SerialName("twitter")
data class Twitter(
	override val common: Common,
	@SerialName("tweets")
	val tweets: List<...>
) : Source()

Obviously this fails because of the discriminator . Is there an elegant way where I can serialize/deserialize given these data classes? Or must I follow another different approach?

[chr]

Are you using the right Json instance for (de)serializing?

val json = Json {
  classDiscriminator = "discriminator"
}

val user: User = json.decodeFromString(myJsonString)

[gts13]

yes I do but it still fails

Polymorphic serializer was not found for missing class discriminator ('null')

I assume that happens because my Source does not contain the discriminator val or am I mistaken?

edit: it fails because the discriminator in the json file lives outside of the Source object

[gts13]

and this is what I try to deal with

[gts13]

it's not like this example
https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-class-discriminator/

[ephemient]

easier to have an intermediate

@Serializable
data class SourceSurrogate(
    val discriminator: String,
    val facebook: Facebook? = null,
    val instagram: Instagram? = null,
    val twitter: Twitter? = null,
)

which you then either map manually or use https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/serializers.md#composite-serializer-via-surrogate on

[gts13]

thank you @ephemient, that worked! 🤝