FederatedTypeResolver

@KeyDirective(FieldSet("id"))
data class MySubThing(val id: Int?, val extraThing: String)

data class MyThing(@ExtendsDirective val id: String, val subThings: List<MySubThing>) {
}

class MySubThingResolver: FederatedTypeResolver<MySubThing> {
    override suspend fun resolve(
      environment: DataFetchingEnvironment,
      representations: List<Map<String, Any>>
    ): List<MySubThing?> {
      return representations.stream().map {
        MySubThing(it["id"]?.toString()?.toInt(), "WOOOP")
      }.collect(Collectors.toList())
    }

graphql.federation.enabled

true

FederatedTypeRegistry

@Bean
  fun federatedTypeRegistry(federatedTypeResolvers: List<FederatedTypeResolver<*>>): FederatedTypeRegistry {
    return FederatedTypeRegistry(federatedTypeResolvers.map { "MySubThing" to it }.toMap())
  }

@key

MyThing

@KeyDirective(fields = FieldSet("id"))
@ExtendsDirective
class MyThing(@ExternalDirective val id: String, val subThings: List<MySubThing>)