Polymorphic kotlinx serialization for complex hierarchy

Question

With polymorphic serialization, I want to serialize classes A, B1, B2. I want to get the following line with class B2

{"type":"type_2","baseField":"base_field_B","fieldB":true,"fieldB2":"field_B_2"}

I use hierarchy like

interface BaseClass {
    val baseField: String
}

@Serializable
@SerialName("type_1")
data class A(
    override val baseField: String,
    val fieldA: Int
) : BaseClass

@Serializable
@SerialName("type_2")
abstract class B: BaseClass{
    abstract override val baseField: String
    abstract val fieldB: Boolean
}

@Serializable
data class B1(
    override val baseField: String,
    override val fieldB: Boolean,
    val fieldB1: Float
) : B()

@Serializable
data class B2(
    override val baseField: String,
    override val fieldB: Boolean,
    val fieldB2: String
) : B()

and doing serialization like this

Json(context = SerializersModule {
    polymorphic(BaseClass::class) {
        A::class with A.serializer()
        B::class with B.serializer()
    }
}).stringify(
    PolymorphicSerializer(BaseClass::class),
    B1("base_field_B", true, "field_B_2")
)

As a result, I get kotlinx.serialization.SerializationException: class B2 (Kotlin reflection is not available) is not registered for polymorphic serialization in the scope of interface BaseClass (Kotlin reflection is not available)

How do I serialize a given structure correctly? Classes B1, B2 must be serialized with type "type_2".

Nikky Nikky · Accepted Answer · 2019-10-21T17:24:34

for what you want ... maybe optional String and Float fields would suffice as a workaround to merge B, B1 and B2 into one class?

@Serializable
@SerialName("type_2")
data class B(
    override val baseField: String,
    val fieldB: Boolean,
    val fieldB1: Float? = null,
    val fieldB2: String? = null
): BaseClass

As far as the Serializer is concerned you have not registered B1 and B2 and you cannot use the same type "type_2" since deserialization will be impossible, it is not smart enough to use a presence or absence of keys to patternmatch the right serializer to serialize and deserialize to the correct type they need to have a different type value on each type (A, B, B1, B2) and all need to be serialized

Polymorphic kotlinx serialization for complex hierarchy

1 Answers