class MyTest {

    @Serializable
    sealed class MyClass {
       abstract val myInt: Int
       abstract val myString: String
    }

    @Serializable
    data class MyClass1(override val myInt: Int, override val myString: String, val myLong: Long) : MyClass()

    @Serializable
    data class MyClass2(override val myInt: Int, override val myString: String, val myInstant: Instant) : MyClass()

    @Test
    fun storeAndRetrieveSealedClasses() = runTest {
       val client = MongoClient.create("mongodb://localhost")
       val mongoDatabase = client.getDatabase("MyTest")

       val document1 : MyClass = MyClass1(0, "a", 3)
       val document2 : MyClass = MyClass2(0, "b", Instant.parse("2023-01-01T12:13:14Z"))

       // Sanity test
       assertThat(Json.decodeFromString<MyClass>(Json.encodeToString(document1))).isEqualTo(document1)
       assertThat(Json.decodeFromString<MyClass>(Json.encodeToString(document2))).isEqualTo(document2)

       val c = mongoDatabase.getCollection<MyClass>("Test")

       c.insertOne(document1)
       c.insertOne(document2)

       val result = c.find().toList() // This line throws
    }
} 

kotlinx.serialization.SerializationException: Missing required discriminator field `_t` for polymorphic class: `MyTest.MyClass`.