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jonaskoelker-jypo avatar jonaskoelker-jypo commented on September 22, 2024 1

My semi-systematic experimentation suggests that the observed field order will be:

  • all non-Option fields, in declaration order (relative to one another); then
  • all Option fields, in declaration order (relative to one another)

By declaration order, I mean the same order as productElementNames.

So if either all fields are Options or zero fields are Options, the existing code will do the right thing.

Here's a blueprint for a workaround:

def workaround[T <: Product](w: OWrites[T]): OWrites[T] = {
  OWrites.transform(w) { case (product, misorderedObject) =>
    val pairs = misorderedObject.value
    JsObject(
      product.productElementNames
        .map(key => pairs.get(key).map(key -> _))
        .collect { case Some(pair) => pair }
        .toSeq
    )
  }
}

implicit val writes: OWrites[Foo] = workaround(Json.writes[Foo])

This can be simplified—you can use apply instead of get and then remove the collect—if all fields are always present in the output, e.g. if you use JsonConfiguration(optionHandlers = OptionHandlers.WritesNull) in a relatively simple context.

It is of note that the bug seems to still occur if all fields are present in the output—so it must depend on the optionality in the declaration, not in the output.

In version 3.0.3 I did some jump-to-source maneuvers in my application:

That sounds suspicious, but it's no smoking gun. I dug around some more, and found this:

This looks like the cause of exactly the pattern I have observed, i.e. a smoking gun.

Across my work I think I have only seen the bug in those projects that use Scala 3. This makes sense, the apparent smoking gun is in a scala3-specific folder, so one would expect the bug to occur if and only if using Scala 3. This matches what @levinson wrote in the issue-creating post: "TODO: Test fails when using Scala 3".

I hope this helps.

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gbarmashiahflir avatar gbarmashiahflir commented on September 22, 2024

Happens here as well.
Having for example this class:

object MetricStatistics{
  implicit val format: Format[MetricStatistics] = Json.format[MetricStatistics]
}

case class MetricStatistics(from: Instant, to: Instant, min: Option[Double], max: Option[Double], average: Option[Double], count: Long, centroid: Option[GeoLocation])

In scala 2.13, generated json was according to field order, but after upgrading to Scala 3, this is the order of an example:

    {
        "count": 6,
        "min": 10.3,
        "to": "2010-06-24T09:55:00Z",
        "max": 100,
        "from": "1980-06-24T05:55:00Z",
        "centroid": {
            "lat": 0,
            "lon": 0
        },
        "average": 38.926
    }

It breaks a lot of tests we have :(
Is there a fix in the plan?

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