Ktor returns 415 from endpoints where receive() is used with ContentNegotiation - kotlin

Question

I have parameter classes with the #Searializable annotation:
#Serializable
data class ShowPostURL(
val date: String,
val titleSlug: String,
override val redirectTo: String? = null
)
and no matter what I do call.receive() won't work. I'm getting HTTP 415 errors and Ktor doesn't log anything. I've added the serialization support as well:
install(ContentNegotiation) {
json()
}
How do I fix this? This is how I'm trying to use it:
accept(ContentType.Any) {
get("/foo/{date}/{titleSlug}") {
val input = call.receive(ShowPostURL::class)
call.respondText("foo")
}
}
If I do a trace I can see that my route is matched, but it can't receive the parameters. Is this json() setup is supposed to work when I'm deserializing from url parameters like this?

Answer 1

Firstly, ContentNegotiation feature works only for receiving custom objects from the payload of POST, PUT and PATCH requests:
POST, PUT and PATCH requests have an associated request body (the payload). That payload is usually encoded.
In order to receive custom objects from the payload, you have to use the ContentNegotiation feature. This is useful for example to receive and send JSON payloads in REST APIs.
When receiving, the Content-Type of the request will be used to determine which ContentConverter will be used to process that request
Secondly, there are three out of the box ContentConverter available:
GsonConverter, JacksonConverter and SerializationConverter.
Each of these converters has its own configuration function: gson, jackson and serialization respectively. You use json configuration function which is most likely is not appropriate for the configuration of ContentNegotiation.
To solve your problem you can access URL parameters by referring them with call.parameters and manually create ShowPostURL object. Then serialize it with the kotlinx.serialization framework if needed.
Also, you can write your own ContentConverter to implement custom logic for receiving typed objects.