I could use some help on a tricky serialization is...

# serialization

[Justin]

I could use some help on a tricky serialization issue. For context, I am using the current version of kotlinx.serialization in a multplatform project.

The basic question is this: what is the best way to handle deserialization of Json when one of the field names (i.e. keys) is variable?

I have put together a simple example to illustrate the problem:

@Serializable data class UserData(val name: String)

@Serializable
data class User(
    val user_data: UserData, 
    val user_contributions: List<Contribution>
) {}

interface Action {
    val id: String
}

@Polymorphic interface Content

@Polymorphic interface Contribution : Action {
    val content: Content
}

@Serializable data class Mentor(
    override val id: String,
    
    @SerialName("mentorContent")
    override val content: MentorContent
) : Contribution {
    @Serializable data class MentorContent(val mentor: String, val mentee: String) : Content
}

@Serializable data class Judge(
    override val id: String,
    
    @SerialName("judgeContent")
    override val content: JudgeContent
) : Contribution {
    @Serializable data class JudgeContent(val judge: String, val judged: String) : Content
}

// Given this JSON:

{
    "user_data": {
        "name": "Justin"
    },
    "user_contributions": [
        {
            "id": "1",
            "mentorContent": {
                "mentor": "Lisa",
                "mentee": "Phil"
            }
        },
        {
            "id": "2",
            "judgeContent": {
                "judge": "Robert",
                "judged": "Karen"
            }
        }
    ]
}

// This is the desired deserialized object:

User(
    user_data = UserData(name = "Justin"),
    user_contributions = listOf(
        Mentor(
            id = "1",
            content = MentorContent(mentor = "Lisa", mentee = "Phil")
        ),
        Judge(
            id = "2",
            content = JudgeContent(judge = "Robert", judged = "Karen")
        )
    )
)

[Fudge]

1. interfaces don't need @polymorphic afaik
2. Can the content of user_contributions be anything? or is it just a finite set of keys

[Fudge]

If the former, you use a map, if the latter, what you did is pretty good. However, if the finite set is pretty big (like 5+) or likely to change in the future, you should consider using a map instead.

[Justin]

It’s a finite set of keys (each of which maps to a corresponding data class)

[Justin]

In my codebase, there are ~80 possible models (and keys) for that content property

[Justin]

@Fudge When you say use a map, do you mean a custom serializer?

[Fudge]

No. But you might need a custom serializer for that id. With no id you could do

user_contributions: Map<String,Map<String,String>>

[Fudge]

I can take a look at this later

[Justin]

Oh I see what you’re saying, but that’s not actually solving my problem. Just to be clear, I don’t want to serialize user_contributions into a generic map; it must to be serialized into a list of type Contribution (hence all the types I wrote out).

[molikuner]

I solved this issue with a custom serializer but I'm not sure whether or not this is the only solution. I wrote a serializer for the common interface (mine was an abstract/sealed class). (in your case that would be Contribution) Downside: You need to repeat all the possible types.

[Justin]

@molikuner When you say you need to repeat all the types, you mean you need to switch on the key string to find a matching type?

[molikuner]

Yes, something like that. I mean you could probably do that via reflection but I prefer(red) the when statement.