Scala play json combinators for validating equality

Question

I'm using play 2.2.0 Reads for validating incoming request in my application.

I'm trying to implement a very simple thing with json API. I have a json like this:

{
  "field1": "some value",
  "field2": "some another value"
}

I already have Reads that checks for other stuff like minimal length

case class SomeObject(field1: String, field2: String)
implicit val someObjectReads = (
  (__ \ "field1").read(minLength[String](3)) ~
  (__ \ "field2").read(minLength[String](3))
)(SomeObject)

I want to create a parser combinator that will match values of two fields and return JsSuccess if values are equal and otherwise JsError and combine it with existing Reads.

How can I achieve this?

UPDATE: clarified the question and changed the code.

Answer 1

You can use filter to do further validation on parsed values:

import play.api.data.validation._
import play.api.libs.json._
import play.api.libs.functional.syntax._
import play.api.libs.json.Reads._

case class SomeObject(field1: String, field2: String)
implicit val someObjectReads = (
  (__ \ "field1").read(minLength[String](3)) ~
  (__ \ "field2").read(minLength[String](3))
)(SomeObject).filter(
  ValidationError("field1 and field2 must be equal")
) { someObject =>
  someObject.field1 == someObject.field2
}

If you want the error message to be listed against each field, then you'll have to use flatMap, eg:

implicit val someObjectReads = (
  (__ \ "field1").read(minLength[String](3)) ~
  (__ \ "field2").read(minLength[String](3))
)(SomeObject).flatMap { someObject =>
  Reads { _ =>
    if (someObject.field1 == someObject.field2) {
      JsSuccess(someObject)
    } else {
      JsError(Seq(
        JsPath("field1") -> Seq(ValidationError("field1 and field2 must be equal")),
        JsPath("field2") -> Seq(ValidationError("field1 and field2 must be equal"))
      ))
    }
  }
}