Play JSON: turn a Seq[Reads[JsObject]] into one Reads[JsObject]

Question

I dynamically generate a bunch of Reads[JsObject] which I then have in a Seq[Reads[JsObject]]. In order to actually apply all these single Reads[JsObject], I've got to merge them with and into one single Reads[JsObject]. Is this possible?

I've got (example):

val generatedReads: Seq[Reads[JsObject]] = Seq(
    (__ \ "attr1").json.copyFrom((__ \ "attr1" \ "attr1a").json.pick),
    (__ \ "attr2").json.pickBranch
)

What I need:

val finalReads: Reads[JsObject] =
    (__ \ "attr1").json.copyFrom((__ \ "attr1" \ "attr1a").json.pick) and
    (__ \ "attr2").json.pickBranch

The property names and which branch to pick is not known at compile time, that's why it's got to be dynamic.

andrey.ladniy · Accepted Answer · 2016-04-28T07:14:36.200

This is a fairly common question. This answer inspired by Reads.traversableReads[F[_], A].

To support the idea of cumulative of Reads[A], we must try all your genererated Reads[JsObject] and we will use Either[Errors, Vector[JsObject]] for this. In original 'Reads.traversableReads[F[_], A]' returns Reads[List[A]] or some collection, but we need simple Json, no problem, ++ concatinates our JsObjects.

def reduceReads(generated: Seq[Reads[JsObject]]) = Reads {json =>
  type Errors = Seq[(JsPath, Seq[ValidationError])]

  def locate(e: Errors, idx: Int) = e.map { case (p, valerr) => (JsPath(idx)) ++ p -> valerr }

  generated.iterator.zipWithIndex.foldLeft(Right(Vector.empty): Either[Errors, Vector[JsObject]]) {
    case (acc, (r, idx)) => (acc, r.reads(json)) match {
      case (Right(vs), JsSuccess(v, _)) => Right(vs :+ v)
      case (Right(_), JsError(e)) => Left(locate(e, idx))
      case (Left(e), _: JsSuccess[_]) => Left(e)
      case (Left(e1), JsError(e2)) => Left(e1 ++ locate(e2, idx))
    }
  }
    .fold(JsError.apply, { res =>
      JsSuccess(res.fold(Json.obj())(_ ++ _))
    })
}

scala> json: play.api.libs.json.JsValue = {"attr1":{"attr1a":"attr1a"},"attr2":"attr2"}

scala> res7: play.api.libs.json.JsResult[play.api.libs.json.JsObject] = JsSuccess({"attr1":"attr1a","attr2":"attr2"},)

new awesome simple answer

After few days I had this brilliant idea. object Reads have implicit Reducer[JsObject, JsObject] so we can reduce Seq(Reads[JsObject]) with FunctionalBuilder (and and then reduce)!

def reduceReads(generated: Seq[Reads[JsObject]]) = 
  generated.foldLeft(Reads.pure(Json.obj())){
    case (acc, r) =>
      (acc and r).reduce 
  }

This solution is simple and clear. Original idea based Seq(Reads[JsObject]) => Seq(JsResult[JsObject]) => Reads[JsObject] mapping, but the last based on basic Json combinators principles Seq(Reads[JsObject]) => Reads[JsObject]

In general, the problem is solved, but the task itself is not correct. If you don't control Reads, what you want see if the same path will be used twice?

I've got to admit, right now I don't fully understand it, but it definitely does what I wanted. I'm sure, the more I play around with the code, I'll figure it out. Thanks! — Nick, Apr 26 '16 at 11:46

Play JSON: turn a Seq[Reads[JsObject]] into one Reads[JsObject]

1 Answers1