如何将验证添加到Play Json中的Reads中？

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.