Is there a way I can use lenses to decide which ob...

# http4k

[Arnab]

Is there a way I can use lenses to decide which object types to create from a response? Say I have a list attachments of type Attachment but it can be either a FileAttachment og PullRequest or even something else. I can make lenses for both subtypes and the supertype but I don’t understand how to check if it is one or the other? To be clear, the two types are mixed in the response.

[Arnab]

I mean..I can just take the id and isUpload field and discriminate between them, and then make a separate response to get them one by one, but that sounds extremely tedious and bad 😞

[dave]

Not with lenses, but you can configure this at the level of the Json library. Assuming Jackson - you can make it do polymorphic deserialisation based on a discriminator field.

[Arnab]

Do you have an example in the docs somewhere? Or is this strictly jackson specific (or rather json library specific) ?

[s4nchez]

@Arnab do you have an example of your use case? By mentioning upload I suspect you may be referring to multipart bodies, in which case you can refer to these docs for examples of how to deal with them (including type safe approach)

[Arnab]

So, my usecase is as follows:
We are looking into migrating our trello board into github issues.
I actually think http4k is a really good tool for this as it allows me to take a bunch of responses from the trello api and check that type conversion works properly before migrating anything into github.

Trello issues might have multiple types of attachments:
◦ files of type image/jpeg, image/png, .mov
◦ github PR (mostly a link to a github PR)
◦ ..other types of attachments that I haven’t thought about.
For now, I ended up doing this:

callTrelloApi("cards/${id}/attachments").let {
        it.bodyString().asJsonObject().map { attachment ->
            attachment["isUpload"].booleanValue() to
                    Response(Status.OK).with(Body.json().toLens() of attachment)
        }.map { (isUpload, attachmentResponse) ->
            if (isUpload) fileAttachmentLens(attachmentResponse) else githubPRLens(attachmentResponse)
        }
    }

This sorta works, but I do wish there was a way to write :

val attachmentLens = Body.auto<Attachment>().toLens()

and then link it somehow to the lenses I created for FileAttachment and GithubPRAttachment but doesn’t seem to be the case 😛

[Andrew O'Hara]

Jackson polymorphic deserialization is pretty simple if you have a dedicated "type" field you can check.

I don't have any OSS examples, but this SO answer shows a Java example. Then you can just use a lens for the abstract class. If you're using http4k-contract, you may need to use JsonTypeInfo.As.EXISTING_PROPERTY and add the property to the base class rather than it be implied.
https://stackoverflow.com/a/30386694/1253613

[Andrew O'Hara]

Oh what the heck. This DTO example is in a public API anyway.

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property = "type")
@JsonSubTypes(
    JsonSubTypes.Type(value = IntOptionsDtoV5.Range::class, name = "range"),
    JsonSubTypes.Type(value = IntOptionsDtoV5.List::class, name = "list"),
)
sealed class IntOptionsDtoV5(val type: String) {
    data class Range(val min: Int, val max: Int): IntOptionsDtoV5("range")
    data class List(val values: kotlin.collections.List<Int>): IntOptionsDtoV5("list")

    companion object {
        val lens = Jackson.autoBody<IntOptionsDtoV5>().toLens()
    }
}

[Arnab]

Interesting, maybe it can discriminate based on a Boolean too. Thanks! I appreciate it!