Unions

Marker interfaces (i.e. interfaces without any common fields or methods) are exposed as GraphQL union types. All the types that are implementing this marker interface and are available on the classpath will be automatically exposed as objects in the target schema.

interface BodyPart

data class LeftHand(val field: String): BodyPart

data class RightHand(val property: Int): BodyPart

class PolymorphicQuery {
    fun whichHand(whichHand: String): BodyPart = when (whichHand) {
        "right" -> RightHand(12)
        else -> LeftHand("hello world")
    }
}

The above will generate following GraphQL schema

union BodyPart = LeftHand | RightHand

type LeftHand {
  field: String!
}

type RightHand {
  property: Int!
}

type Query {
  whichHand(whichHand: String!): BodyPart!
}