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?

score 0 · Accepted Answer · answered Sep 05 '21 at 02:25

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

I have added the @BsonDiscriminator annotation, and have done other thing like registering codec, but it is still not working. the kmongo official website document is not so clear enough for me — user16799920, Sep 06 '21 at 08:51
@user16799920 can you share your kmongo dependency or project? I had tried out all 3 before answering. The `@BsonDiscriminator` will only work if you are using `-native` dependency. Registering a codec should not be necessary unless there's something in `Data` class that we're not seeing. — Aarjav, Sep 07 '21 at 22:57
It turns out that I was not using the -native dependency. Genuinely sorry for my carelessness — user16799920, Sep 08 '21 at 18:01

Kotlin Kmongo library with abstract class collection

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