Deserialize object from string - kotlin

Question

I'm trying to deserialize object from string, but get "no String-argument constructor/factory method to deserialize from String value ('test')" exception.
import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
import com.fasterxml.jackson.module.kotlin.readValue
import io.kotest.core.spec.style.FunSpec
import io.kotest.matchers.shouldBe
class KotlinJacksonTest : FunSpec({
val mapper = jacksonObjectMapper()
test("deserialize as string") {
class Ref #JsonCreator constructor(#JsonValue val name: String)
class Root(val ref: Ref)
val root = mapper.readValue<Root>(""" { "ref": "test"} """)
root.ref.name shouldBe "test"
}
})
What I need is to make jackson serialize my object as if it was a string.
But it constantly fails with the following error:
Cannot construct instance of `KotlinJacksonTest$1$1$Ref` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('test')
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `KotlinJacksonTest$1$1$Ref` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('test')
at [Source: (String)" { "ref": "test"} "; line: 1, column: 11] (through reference chain: KotlinJacksonTest$1$1$Root["ref"])
What I'm doing wrong? I can clearly remember this worked for me in java when I needed to do same but with map instead of string.

Answer 1

Ok, after reading some documentation on JsonCreator annoation turns out that it must be like this:
class Ref #JsonCreator(mode = JsonCreator.Mode.DELEGATING) constructor(#JsonValue val name: String)
The JsonCreator.Mode.DELEGATING makes it work like I wanted.
UPD: There is even a possibility to make it shorter - custom jackson annotation.
#Retention(AnnotationRetention.RUNTIME)
#Target(AnnotationTarget.CONSTRUCTOR)
#JacksonAnnotationsInside
#JsonCreator(mode = JsonCreator.Mode.DELEGATING)
annotation class JsonDelegating
And usage:
class Ref #JsonDelegating constructor(#JsonValue val name: String)

Answer 2

You can avoid having to annotate the constructor of your data class completely by just targeting the #JsonValue annotation on your property to its getter method, like so:
class Ref(#get:JsonValue val name: String)
Notably #field:JsonValue does not work in this case either.
Why your original code, nor the field targeting, do not work as expected, I couldn't tell you.