Serializer for producing json diffs between two versions of the same object

[Kantis]

From discussions in in the Kotlin serialization slack channel..

Some APIs offer / require use of PATCH inputs, where you send a PATCH request of the fields you want to change. It could be convenient if we could produce such a delta, given two instances of an object.

data class Bar(val baz: String, val b: Int)
data class Foo(val x: Int, val y: String, val z: Bar)

val first = Foo(5, "y", Bar("a", 10)) 
val second = Foo(6, "y", Bar("b", 10))

Given the above, we want to produce the json

{
  "x": 6,
  "z": {
    "baz": "b"
  }
}

I'm not entirely sure how/if we could achieve it with regular serializers to be honest.

[sschuberth]

I'm curious about what this is exactly about, could you add some more details to your top post? Like, where do the old and new object instances come from?

[Kantis]

clarified top post. let me know what you think :)

[sschuberth]

Interesting, but probably a bit out-of-scope. Anyway, maybe we could have a built-in class similar to

data class Diff<T>(
    val before: T,
    val after: T
)

that, when serialized, only serializes the fields that differ in after from before. The challenge probably is to implement this without the use of reflection... I was thinking whether there's a trick to leverage the built-in string representation of the data class, but that idea was not getting me ahead.

[Kantis]

Oh, neat idea! I think if we use the default serializer to serialize both before and after to json-trees, then we could traverse and compare them in a JsonTransformingSerializer before emitting the final json..