I'm using Enums a fair amount in my (data class) m...

# serialization

[Don Kittle]

I'm using Enums a fair amount in my (data class) model properties in commonMain (Kotlin MPP). I'm trying to serialize them to JSON. I get the following error when doing so: Enums cannot be serialized polymorphically with 'type' parameter. You can use 'JsonBuilder.useArrayPolymorphism' instead

My serialization is set up as follows:

val JSON = Json {
    ignoreUnknownKeys = true
    encodeDefaults = true
    explicitNulls = false
}

My enums follow this kind of pattern:

@Serializable
@JsonClassDiscriminator("ItemProperty")
sealed interface ItemProperty {
    @Serializable
    @SerialName("ArmorProperty")
       enum class ArmorProperty : ItemProperty {
        @SerialName("Light") Light,
        @SerialName("Medium") Medium,
        @SerialName("Heavy") Heavy,
        @SerialName("Shield") Shield;
    }

    @Serializable
    enum class FocusProperty : ItemProperty {
        @SerialName("Arcane") Arcane,
        @SerialName("Druidic") Druidic,
        @SerialName("HolySymbol") HolySymbol;
    }
}

I cannot find anything except a really simple enum example in the serialization documentation.
Does anyone know what I'm doing wrong here? I am unsure what ``JsonBuilder.useArrayPolymorphism` ` is or how to use it to resolve this issue.

[ephemient]

Json {
    useArrayPolymorphism = true
}

changes the format of polymorphism

[ephemient]

basically when you have

@Serializable
sealed interface Base {
    @Serializable
    data class Impl(val value: String)
}

Impl.serializer() serializes Impl(value = "foo") as {"value": "foo"}, and JSON's default polymorphic serialization of Base takes that and adds a type discriminator, e.g. {"type": "Impl", "value": "foo"}. that's only possible with classes/objects, not enums

[ephemient]

useArrayPolymorphism uses a wrapper array instead of adding a property to the object, e.g. ["Impl", {"value": "foo"}], which does work for non-objects

[Don Kittle]

Is there a way to only use ArrayPolymorphism on the interface/enums and use the Polymorphism serialization for the data class that uses enums in some of it's properties? If I just the ArrayPolymorphism for the whole data class (plus it's enum properties), I get an array of one instance of the data class - and that is causing Kmongo to complain...

One of my data classes looks like this, for example:

@Serializable
data class Gear(
    val name: String,
    val description: String,
    ...
    val properties: List<ItemProperty>
)

[Don Kittle]

Can I so something like:

@Serializable (with = ArrayPolymorphism)
sealed interface ItemProperty {
...
}

And leave my main json { } config how it was?
(sorry, I'm just guessing here)

[ephemient]

no there isn't

[ephemient]

you could write a custom serializer that does something similar

[Don Kittle]

Ok, I'll give that a try.

[ephemient]

this won't be as safe as the generated polymorphic sealed serializer but should work

Untitled.cpp

[ephemient]

or this, if you want to avoid registering every type manually, relying on reflection instead

Untitled.kt

[Don Kittle]

Thanks, this is helping a lot! I'm having to use the version of registering every type manually as class.sealedSubclasses doesn't resolve in common code (only in jvm code).

[ephemient]

ah, and even if KClass.sealedSubclasses worked, KClass.serializer() wouldn't, since that also relies on JVM reflection

[Don Kittle]

I can serialize with this perfectly. the result is this:

@Serializable
data class SimpleGear(
    val name: String,
    val description: String,
    val weight: Int,
    val properties: List<ItemProperty>
)
val longsword = SimpleGear(
    "Longsword",
    "Hack off heads with this",
    3,
listOf(ItemProperty.WeaponProperty.Slashing, ItemProperty.WeaponProperty.Versatile))

val jsonString = JSON.encodeToString(longsword)

println(jsonString)

println("${longsword == JSON.decodeFromString(jsonString)}")

{
  "name": "Longsword",
  "description": "Hack off heads with this",
  "weight": 3,
  "properties": [
    [
      "ca.kittle.model.reference.ItemProperty.WeaponProperty",
      "Slashing"
    ],
    [
      "ca.kittle.model.reference.ItemProperty.WeaponProperty",
      "Versatile"
    ]
  ]
}

[Don Kittle]

But when I try to deserialize I get:

Exception in thread "main" kotlinx.serialization.SerializationException: Serializer for class 'Any' is not found.
Please ensure that class is marked as '@Serializable' and that the serialization compiler plugin is applied.

[Don Kittle]

ah, and even if KClass.sealedSubclasses worked, KClass.serializer() wouldn't, since that also relies on JVM reflection

[ephemient]

you need to explicitly

JSON.decodeFromString<SimpleGear>(jsonString)

or

JSON.decodeFromString(SimpleGear.serializer(), jsonString)

since the type is inferred as Any in the string

[Don Kittle]

Thanks SO much!

[Don Kittle]

This help has been invaluable.