Support for abstract / polymorphic classes

[williamboxhall]

According to the kotlinx serialization docs it should be possible to register the subtypes of an abstract class as part of the serializerModule. However, it doesn't seem to work:

@file:UseContextualSerialization
...
    val module = SerializersModule {
        polymorphic(SkillsCoachEvent::class) {
            subclass(AccountSettingsEvent::class)
        }
    }
    val schema = Avro(configuration = AvroConfiguration(SnakeCaseNamingStrategy), serializersModule = module).schema(Event.serializer())

...

@Serializable
data class Event(
    val eventId: UUID,
    val createdAt: DateTime,
    val aggregateId: UUID,
    val sequence: Int,
    val aggregateType: String,
    val eventType: String,
    val metadata: ManagerLabMetadata,
    @Polymorphic @Contextual val body: SkillsCoachEvent
)

...

@Serializable
@Polymorphic
abstract class SkillsCoachEvent : DomainEvent

As you can see, I also naively tried adding both @Polymorphic and @Contextual and @file:UseContextualSerialization clutching at straws that they'D offer the hints necessary for Avro4K to treat this as SerialKind.CONTEXTUAL instead of SerialKind.OPEN, to no available. I suspect that Avro4K might need to introduce a new SerialKind.POLYMORPHIC or something like that, to match what's in kotlinx serialization.

Exception in thread "main" kotlinx.serialization.SerializationException: Unsupported type kotlinx.serialization.Polymorphic<SkillsCoachEvent> of OPEN
    at com.github.avrokotlin.avro4k.schema.SchemaForKt.schemaFor(SchemaFor.kt:206)
    at com.github.avrokotlin.avro4k.schema.SchemaForKt.schemaFor(SchemaFor.kt:187)
    at com.github.avrokotlin.avro4k.schema.ClassSchemaFor.buildField(ClassSchemaFor.kt:78)
    at com.github.avrokotlin.avro4k.schema.ClassSchemaFor.dataClassSchema(ClassSchemaFor.kt:63)
    at com.github.avrokotlin.avro4k.schema.ClassSchemaFor.schema(ClassSchemaFor.kt:43)
    at com.github.avrokotlin.avro4k.Avro.schema(Avro.kt:264)
    at com.github.avrokotlin.avro4k.Avro.schema(Avro.kt:269)
    at events.AvroSchemaGeneratorKt.main(AvroSchemaGenerator.kt:37)

[williamboxhall]

I tried to work around this with @org.apache.avro.reflect.Union but it didn't help

[williamboxhall]

As mentioned in https://github.com/avro-kotlin/avro4k/issues/101#issuecomment-890573467 @thake says OPEN polymorphic references aren't supported in Avro4k. @thake are there any plans to change this? Can you think of a workaround for this short of making this sealed (sadly not a great option in our codebase)

[thake]

It is planned to support this feature. It should not be too hard to implement. @williamboxhall If you want to give it a try, I will be very happy to review a PR for this feature.

[williamboxhall]

Thanks @thake that's a fair call. I'm not sure if I'll be able to absorb getting up to speed in avro4ks internals and then doing this PR while dealing with our internal deadlines, but I will definitely try :).

In the meantime, our workaround is probably to just use properly sealed classes, which causes some other significant compromises but gives us a path forward.

[williamboxhall]

I've taken a stab at making a PR for this: https://github.com/avro-kotlin/avro4k/pull/109

[thake]

Implemented with #110