Equivalent of Jackson's JsonUnwrapped in kotlinx.serialization - kotlin

Question

I have a class such as:
#Serializable
data class Example(
val id: Int
val items: Map<String, Int>
)
val instance = Example(1, hashMapOf("one" to 1, "two" to 2))
How do I make Kotlin serialization lift the keys of the items map into the Example object during serialization? If I was using Jackson I would annotate the items property with #JsonUnwrapped.
I've tried the default serialization and the items property is its own key with the hashmap represented as an object:
{
"id": 1,
"items": {
"one": 1,
"two": 2
}
}
whereas I would like the object to be serialized like so:
{
"id": 1,
"one": 1,
"two": 2
}

Answer 1

You can use this :
object SortedMapSerializer: KSerializer<Map<String, Int>> {
private val mapSerializer = MapSerializer(String.serializer(), Int.serializer())
override val descriptor: SerialDescriptor = mapSerializer.descriptor
override fun serialize(encoder: Encoder, value: Map<String, Int>) {
mapSerializer.serialize(encoder, value.toSortedMap())
}
override fun deserialize(decoder: Decoder): Map<String, Int> {
return mapSerializer.deserialize(decoder)
}
}
#Serializable
class Example(
#Serializable(with = SortedMapSerializer::class)
val map: Map<String, Int>
)
fun main() {
val example = Example(mapOf("b" to 2, "c" to 3, "a" to 1))
println(Json.encodeToString(example))
}