How do you add validation to a Reads in Play Json? - json

Question

Let's say I've got a reads that creates an object from JSON with two optional fields:
implicit val rd: Reads[MyObject] = (
(__ \ "field1").readNullable[String] and
(__ \ "field2").readNullable[String]
)(MyObject.apply _)
I want to check to make sure that the value of field1 is one of the values in the list:
List("foo", "bar")
I can do that after the fact, by creating a new MyObject and mapping the values through a function to transform them, but I feel like there should be a way to do this more elegantly using JSON transformers or something.
Ideally, I want the Reads to read the nullable value of field1 and transform it if it is defined, without the need to post-process it. Is there some way of sneaking a transform in there?

Answer 1

You can use this approach:
case class MyObject(a: Option[String], b: Option[String])
val allowedValues = Seq("foo", "bar")
implicit val reads: Reads[MyObject] = new Reads[MyObject] {
override def reads(json: JsValue): JsResult[MyObject] = {
val a = (json \ "a").asOpt[String].filter(allowedValues.contains)
val b = (json \ "b").asOpt[String]
JsSuccess(MyObject(a, b))
}
}
Usage examples:
scala> Json.parse(""" { "a": "bar", "b": "whatever"} """).validate[MyObject]
res2: play.api.libs.json.JsResult[MyObject] = JsSuccess(MyObject(Some(bar),Some(whatever)),)
scala> Json.parse(""" { "a": "other", "b": "whatever"} """).validate[MyObject]
res3: play.api.libs.json.JsResult[MyObject] = JsSuccess(MyObject(None,Some(whatever)),)
scala> Json.parse(""" {} """).validate[MyObject]
res4: play.api.libs.json.JsResult[MyObject] = JsSuccess(MyObject(None,None),)

Answer 2

Okay, after doing some more research, I came up with the following:
In play.api.libs.json.ConstraintReads there is a function called verifying(cond: A => Boolean) that returns a Reads[A]. This can be passed as a parameter to JsPath.readNullable[A] like so:
implicit val rd: Reads[MyObject] = (
(__ \ "field1").readNullable[String](verifying(allowedValues.contains)) and
(__ \ "field2").readNullable[String]
)(MyObject.apply _)
This will return a JsResponse, either a JsSuccess if "field1" validates, or a JsError if it doesn't validate. It actually fails on an invalid input, rather than just ignoring the input. That's more like the behaviour I wanted.
There are a number of other constraint functions that perform similar tests on the read value, as well.