Play: How to implement vermongo with ReactiveMongo

json, mongodb, playframework, reactivemongo, scala

Solution

Fantastic question; I'd been meaning to learn about Play's JSON transformers for ages, and finally a great situation to try them out.

I should note that I was completely unable to get this to work correctly until I added the all-important `functional.syntax._` import:

import play.api.libs.functional.syntax._

The solution started out from the Gizmo -> Gremlin transformer example on the transformers page; that's where I got the usage of the `and` and `reduce`, which completely transforms (pardon the pun) how the whole thing works.

So here it is:

private def toVersioned(deleted: Boolean) = (__.json.update(
  ( __ \ '_id ).json.copyFrom((__ \ '_id \ '$oid).json.pickBranch) and
  ( __ \ '_id \ '_version).json.copyFrom((__ \ '_version).json.pick) and
    (__ \ '_version).json.update(
      of[JsValue].map { case JsNumber(oldVersion) =>
        if (deleted) JsString(s"deleted:$oldVersion") else JsNumber(oldVersion)
      }
    ) and
    (__ \ '_timestamp).json.put(Json.toJson(LocalDateTime.now)) reduce
  )
  andThen
  ( __ \ '_id \ '$oid ).json.prune
)

Key points:

I couldn't (within a reasonable amount of time) achieve a "move" of the `_id` node in the one transform, hence the first pass copies it "down" one level, and the second pass (after the `andThen`) deletes the old `$oid`. There's almost-certainly a way...

The use of `and` seems to change the scope of the `copyFrom`, allowing the `_version` to be picked out of the top-level correctly - when using `andThen` it seemed to only work if descending into nodes "below" the target

I'm quite happy with the handling of `deleted` here - the `map` seems both Scala- and PlayJSON-idiomatic. The `Of[JsValue]` is necessary because we return either a `JsString` or a `JsNumber` here, and `JsValue` seemed like a suitable superclass

Problem

Original source