JsValue has two methods
def validate[A](implicit rds: Reads[A]): JsResult[A] - Tries to convert the node into a JsResult[T] (Success or Error).
def validateOpt[A](implicit rds: Reads[A]): JsResult[Option[A]] - I suppose it also does the same thing.
In which situation would validateOpt be used? In my opinion, if JsResult fails then I'll get the error in JsError. So what is the point of having an additional layer of Option in JsSuccess as JsSuccess will always contain value after successful conversion of JsValue into the type A?
validateOpt should be used when a null JSON value or missing JSON path is not considered an error. For example, say we have the following model
case class Person(
name: String
employer: Option[String]
)
where employer field is optional as it is perfectly reasonable for a person not to be employed, whilst they always have a name. Then deserialising the following JSON
{
"name": "Picard"
}
should succeed even though employer path is missing. Thus manually defining Reads[Person] would make use of validateOpt like so
implicit val personRead: Reads[Person] = Reads { json =>
for {
name <- (json \ "name").validate[String]
employer <- (json \ "employer").validateOpt[String]
} yield Person(name, employer)
}
Also contrast deserialisation of null, for example
val raw = "null"
val json = Json.parse(raw)
println(json.validate[String])
println(json.validateOpt[String])
should output
JsError(List((,List(JsonValidationError(List(error.expected.jsstring),WrappedArray())))))
JsSuccess(None,)
where we see validateOpt resulted in success.