How to parse JSON empty string as `None` using Play Json?

Question

I have a case class that's a "Bag of Fields" - lots of fields that represent a business entity. For my use case, the distinction between "", null, and the k/v being totally absent from the JSON object is irrelevant to me. In this case class these fields are already Option[String]s, so I'd like all 3 of those cases to collapse into None. I believe the current behavior of the auto-generated Reads is what I want except for the empty string case.

This BOF is likely to change in the future, so "just implement your own Reads" (as suggested here: make play-json read the empty string as None for a type of Option[T]) or something else where I have to re-enumerate all the fields is a non-starter.

I think what I may need is Play's 'Json Transformers'. It appears trivial to write a transformer that removes entries if they have empty values, but what I cant figure out is how to compose it with the auto-generated Reads implementation.

I imagine what I need is some combinator over Reads and transformers, some signature like: (JSON -> JSON, JSON -> T) -> (JSON -> T). Obviously I've found this page: https://www.playframework.com/documentation/2.5.x/ScalaJsonCombinators, but none of the listed combinators does what I want I believe. Could this combinator be easily implemented? I'd be a little out of my type-fu depth but that would be a great solution if I could get some pointers.

Alex · Accepted Answer · 2017-03-27 22:11:19Z

1

Here's what worked for me:

class RemoveEmpty[T] (reader: Reads[T]) extends Reads[T] {
  override def reads(json: JsValue): JsResult[T] = json match {
    case JsObject(underlying) => { 
      reader.reads(JsObject(underlying.filterNot{ case (k, v) => jsonValueEmpty(v) } ))
    }
    case _ => { 
      JsError("Non-JsObj passed to RemoveEmpty")
    }
  }

  def jsonValueEmpty(v: JsValue) = v match {
    case JsNull | JsString("") => true
    case _ => false
  }
}

Then you can just use it like this:

implicit val myTypeReads = new RemoveEmpty(Json.reads[MyType])

answered Mar 27, 2017 at 22:11

Alex

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2 Comments

Alex Over a year ago

FWIW I've never seen the error case hit after this function has seen considerable use - I presume Play is smart somehow, I don't particularly understand the innards of its JSON parsing

David Portabella Over a year ago

so cool! however it does not work with implicit val myTypeReads = new RemoveEmpty(play.api.libs.json.Reads.localDateTimeReads(DateTimeFormatter.ISO_DATE_TIME)); it throws your "Non-JsObj passed to RemoveEmpty" error. any idea?

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