Jackson serialization is not working properly with polymorphic types stored in a collection

Hi,

I’ve found that the current ConfigurableJackson.autoBody implementation would not work with collections of polymorphic types. It would lose the @class field in the serialized json in the below example:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "@class")
@JsonSubTypes(
    JsonSubTypes.Type(value = FirstChild::class, name = "first"),
    JsonSubTypes.Type(value = SecondChild::class, name = "second")
)
sealed class PolymorphicParent
data class FirstChild(val something: String): PolymorphicParent()
data class SecondChild(val somethingElse: String): PolymorphicParent()

class SampleTest {
    @Test
    fun `roundtrip polymorphic object to and from body`() {
        val body = Body.auto<PolymorphicParent>().toLens()

        val firstChild: PolymorphicParent = FirstChild("hello")
        val secondChild: PolymorphicParent = SecondChild("world")

        assertThat(body(Response(OK).with(body of firstChild)), equalTo(firstChild))
        assertThat(body(Response(OK).with(body of secondChild)), equalTo(secondChild))
    }

    @Test
    fun `roundtrip list of polymorphic objects to and from body`() {
        val body = Body.auto<List<PolymorphicParent>>().toLens()

        val list = listOf(FirstChild("hello"), SecondChild("world"))
        
        assertThat(body(Response(OK).with(body of list)), equalTo(list))
    }
}

While the first test(one without being stored in a collection) works, the second test would get:

Caused by: com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Missing type id when trying to resolve subtype of [simple type, class org.http4k.format.PolymorphicParent]: missing type id property '@class'
 at [Source: (String)"[{"something":"hello"},{"somethingElse":"world"}]"; line: 1, column: 22] (through reference chain: java.util.ArrayList[0])

[alphaho]

Great! Thanks for your help on the PR as well! =D

[daviddenton]

Releasing in 3.199.0 😄