kotlinx.serialization: issue with multi-level sealed class

Say you need to insertMany elements of a sealed class from a list, you’ll just pass it and I’d expect the field/key type (or ___type as modified by KMongo) to be present in the root of each document.

Since com.mongodb.internal.operation.Operations::insertMany at line 360 treats the serialization of objects as single elements instead of a whole list, the serializer used is the one of the single elements, thus loosing the generation field type.

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There is a bug with kotlinx.serialization and sealed class with more than one level. Workaround: remove the sealed mention except on the root class:

sealed class OuterSealedClass {
     abstract class InnerSealedClass : OuterSealedClass() {
        class A : InnerSealedClass()
        class B : InnerSealedClass()
    }
    class C : OuterSealedClass ()
}

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see #248