Custom JSON validation constraints in Play Framework 2.3 (Scala)

Question

I managed to implement form validation with custom constraints, but now I want to do the same thing with JSON data.

How can I apply custom validation rules to a JSON par

Answer 1

The simplest way I can think of would use the filter method from Reads.

Let's say we have some User object that will determine if the user name exists:

object User {
    def findByName(name: String): Option[User] = ...
}

You could then construct your Reads like this:

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

case class ExampleCaseClass(username: String, somethingElse: String)

object ExampleCaseClass {
    implicit val exampleReads: Reads[ExampleCaseClass] = (
        (JsPath \ "username").read[String].filter(ValidationError("User does not exist."))(findByName(_).isDefined) and
        (JsPath \ "somethingElse").read[String]
    )(ExampleCaseClass.apply _)
}

Your controller function can be simplified using a json BodyParser and fold:

def postNew = Action(parse.json) { implicit request =>
    request.body.validate[ExampleCaseClass].fold(
        error => BadRequest(JsError.toFlatJson(error)),
        obj => {
            // Do something with the validated object..
        }
    )
}

You could also create a separate Reads[String] that will check if the user exists, and explicitly use that Reads[String] within your Reads[ExampleCaseClass]:

val userValidate = Reads.StringReads.filter(ValidationError("User does not exist."))(findByName(_).isDefined)

implicit val exampleReads: Reads[ExampleCaseClass] = (
    (JsPath \ "username").read[String](userValidate) and
    (JsPath \ "somethingElse").read[String]
)(ExampleCaseClass.apply _)