Using delegated properties in deserialization

[StefanSrf]

Your question
We would like to make use of kotlins 'inheritance by delegation' feature. In the json-structure the object should be flat, i.e. not exposing the delegation. Suppressing the delegation during serialization works, but deserialization fails. Is there a way to achive this or should we raise a feature request?

Example:

class DelegateTest {
    interface SharedInterface {
        val commonString: String
    }

    data class InternalSharedDelegate(override val commonString: String) : SharedInterface

    data class ExposedObject(
        // ignore/hide the delegation in json serialization
        @param:JsonProperty(access = JsonProperty.Access.WRITE_ONLY)
        val delegate: InternalSharedDelegate,
        val otherValue: String
    ) : SharedInterface by delegate

    private val objectMapper = jsonMapper { addModule(kotlinModule()) }

    @Test
    fun `should deserialize delegated property`() {
        val exposedObject = ExposedObject(delegate = InternalSharedDelegate("a"), otherValue = "b")

        val serialized = objectMapper.writeValueAsString(exposedObject)
        assertEquals("{\"otherValue\":\"b\",\"commonString\":\"a\"}", serialized)

        // following line causes:
        // Instantiation of [simple type, class DelegateTest$ExposedObject] value failed for JSON property delegate due to missing (therefore NULL) value for creator parameter delegate which is a non-nullable type
        val deserialized: ExposedObject = objectMapper.readValue(serialized)

        assertEquals(exposedObject, deserialized)
    }
}

[k163377]

First, it seems difficult to solve this with an approach as jackson-module-kotlin.

There doesn't seem to be any way to detect that a class is inheritance by delegation in kotlin-reflect.
I also checked kotlinx-metadata-jvm, but there seems to be no way to identify the property being used for the delegation.
https://github.com/JetBrains/kotlin/blob/master/libraries/kotlinx-metadata/src/kotlinx/metadata/Flag.kt

In other words, there is no way to get information to control serialization and deserialization.

I also checked Jackson features, but could only find unimplemented features.
FasterXML/jackson-future-ideas#28

[k163377]

It is closed as a duplicate of #459.

[sandwwraith]

I was just passing by, but kotlinx-metadata-jvm has a way to determine if a declaration came from interface delegation: it is stored in the kind enum, e.g. kmProperty.kind == MemberKind.DELEGATION (using new API, old Flags API would look like Flags.Property.IS_DELEGATION(prop.flags)

[k163377]

@sandwwraith
Thank you for your comment.
It does indeed seem to work, I may have misunderstood something.

However, I am not considering implementing this feature itself.
Please see my comment below for more details.
#459 (comment)