CodecRegistries.fromRegistries(
    CodecRegistries.fromCodecs(
        KotlinSerializerCodec.create<Polymorphic>(
            serializersModule = defaultSerializersModule + SerializersModule {
                polymorphic(Polymorphic::class) {
                    subclass(Polymorphic1::class)
                    subclass(Polymorphic2::class)
                }
            },
            bsonConfiguration = BsonConfiguration(classDiscriminator = "_t", encodeDefaults = true)
        )
    ),
    CodecRegistries.fromProviders(
        KotlinCodecProvider()
    ),
    Bson.DEFAULT_CODEC_REGISTRY,
)

@Serializable
abstract class Polymorphic {

    abstract val id: ObjectId

}

@Serializable
@SerialName(value = "ONE")
data class Polymorphic1(
    @SerialName(value = "_id")
    @Serializable(with = ObjectIdSerializer::class)
    override val id: ObjectId,
    val a: String,
    val b: String,
    val c: Boolean,
) : Polymorphic()

@Serializable
@SerialName(value = "TWO")
data class Polymorphic2(
    @SerialName(value = "_id")
    @Serializable(with = ObjectIdSerializer::class)
    override val id: ObjectId,
    val d: Boolean,
    val e: Int,
    val f: String,
) : Polymorphic()

val polymorphicCollection = database.getCollection<Polymorphic>("polymorphic")

val registry = polymorphicCollection.codecRegistry[Polymorphic::class.java]

var document = BsonDocument()
var writer = BsonDocumentWriter(document)

val polymorphic1 = Polymorphic1(ObjectId(),"a","b",true)

registry.encode(writer,polymorphic1,EncoderContext.builder().build())

writer.flush()

println(document.toJson(JsonWriterSettings.builder()
    .indent(true)
    .build()
))

document = BsonDocument()
writer = BsonDocumentWriter(document)

val polymorphic2 = Polymorphic2(ObjectId(),true,1,"f")

registry.encode(writer,polymorphic2,EncoderContext.builder().build())

writer.flush()

println(document.toJson(JsonWriterSettings.builder()
    .indent(true)
    .build()
))

runBlocking {
    polymorphicCollection.drop()
    polymorphicCollection.insertOne(polymorphic1)
    polymorphicCollection.insertOne(polymorphic2)
}

{
  "_t": "ONE",
  "_id": {
    "$oid": "6608b5ef80e74b7949643522"
  },
  "a": "a",
  "b": "b",
  "c": true
}
{
  "_t": "TWO",
  "_id": {
    "$oid": "6608b5ef80e74b7949643523"
  },
  "d": true,
  "e": 1,
  "f": "f"
}

testing> db.polymorphic.find().pretty()
[
  {
    _id: ObjectId('6608b5ef80e74b7949643522'),
    a: 'a',
    b: 'b',
    c: true
  },
  {
    _id: ObjectId('6608b5ef80e74b7949643523'),
    d: true,
    e: 1,
    f: 'f'
   }
]