Scala Play 2.6: Parse json with constraint between fields - json

Question

I'd like to validate a field depending on the value of another field when parsing a json.
For example when I'm reading a range, I'd like to verify min < max:
import org.scalatest.{FlatSpec, Matchers}
import play.api.libs.json.{JsError, Json, Reads}
class JsonReadsTest extends FlatSpec with Matchers {
"Json" should "be reads" in {
val reads: Reads[Range] = ???
val json = Json.obj("min" -> 3, "max" -> 2)
reads.reads(json) shouldBe JsError("max should be superior to min")
}
}
case class Range(min: Int, max: Int)

Answer 1

Stright-forward-oop solution
You can create an object that extends Reads[T] and implement reads method directly without fuctional builders
(this approach is not fully covered in docs but you can find many exambles in source code)
val reads1 = new Reads[Range] {
def reads(json: JsValue): JsResult[Range] = {
(for {
min <- (json \ "min").validate[Int]
max <- (json \ "max").validate[Int]
} yield (min, max)).flatMap {
case (min, max) if max > min =>
JsSuccess(Range(min, max))
case _ =>
JsError(Seq(JsPath ->
Seq(JsonValidationError("error.expected.range"))))
}
}
}
Functional-builder solution
val reads2 = (
(__ \ "min").read[Int] and
(__ \ "max").read[Int]
).tupled
.filter(JsonValidationError("error.expected.range")){ case (min, max) => max > min}
.map{ case (min, max) => Range(min, max)}
Best practice
You are trying to mix reading and validation and it can be a problem when you have different validation rules in different modules. There is a practice to parse json object to an presentational-model-class like
case class RangeInput(min: Int, max: Int)
and then convert it to business model class performing validation
def validate(input: RangeInput): Option[Range] =
input.filter(i => i.max > i.min).map(i => Range(i.min, i.max))
If you need to aggregate validation errors things like cats validated can help you with that