Kotlin Kmongo library with abstract class collection

Question

I have these models

abstract class Message : Data
data class TextMessage(val m: String): Message
data class ImageMessage(val m: ByteArray): Message

and I want to get a collection by the abstract class Message

database.getCollection<Message>

But it will actually be a implement class (TextMessage, ImageMessage) instance depending on it's content

when(val value = collection.findOne()) {
    is TextMessage -> {}
    is ImageMessage -> {}
}

how to do this?

Answer 1

The KMongo library has 3 options for object mapping, and your solution will depend on which one is being used.

By default, Jackson engine is used. You can use POJO Codec engine by adding a -native suffix to the artifactId, or Kotlinx Serialization by adding a -serialization suffix to the artifactId.

https://litote.org/kmongo/quick-start/#object-mapping-engine

Depending on the engine used, apply how that engine handles polymorphism:

• Jackson

  @JsonTypeInfo(use = JsonTypeInfo.Id.CLASS)
  abstract class Message
  

  more info: https://github.com/FasterXML/jackson-docs/wiki/JacksonPolymorphicDeserialization

• Native

  @BsonDiscriminator
  abstract class Message
  

  not sure if there is much more info on this but the official doc here: https://mongodb.github.io/mongo-java-driver/4.3/bson/pojos/#annotations

  this essentially stores the class name under a _t field and gets used during deserialization

• kotlinx serialization

  @Serializable
  sealed class Message
  @Serializable
  data class TextMessage(val m: String) : Message()
  @Serializable
  data class ImageMessage(val m: ByteArray) : Message()
  

  Like jackson, there's other ways and things you may want to consider here: https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md