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.
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])
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