Getting better error messages from Play JSON API

Question

Examples of the kind of error messages produced by Play JSON API:

scala> import play.api.libs.json._
import play.api.libs.json._

scala> Json.obj(\"k\" -&g         


        

Answer 1

yes using JsValue.as[T] isn't robust in this case:

res0 \ "p" returns JsUndefined which is then applied to an implicit Reads[String] and it says "hey that's not a String"... That's true but not what you expect ;)

the better way IMHO uses Reads on JsPath directly:

scala> ((__ \ "p").read[String]).reads(res0)
res7: play.api.libs.json.JsResult[String] = JsError(List((/p,List(ValidationError(validate.error.missing-path,WrappedArray())))))

scala> ((__ \ "k" \ "m").read[String]).reads(res0)
res8: play.api.libs.json.JsResult[String] = JsError(List((/k/m,List(ValidationError(validate.error.expected.jsstring,WrappedArray())))))

Here you have a better message about missing-path. It even notifies the missing path in first param in JsError.

Concerning messages as nice strings, you can use a local message to map the key. BTW, those Json errors message are not quite standardized with respect to other error messages in Play such as Forms. I think we will improve it in a future release of Play.

Finally, when you want to send Json validation errors to the client, there is a helper function in JsError to use with JsResult.recoverTotal

scala> ((__ \ "k" \ "m").read[String]).reads(res0).recoverTotal( e => JsError.toFlatJson(e) )
res11: java.io.Serializable = {"obj.k.m":[{"msg":"validate.error.expected.jsstring","args":[]}]}

This is just a raw version of this kind of function. If you need an another format, I advise to look at toFlatJson implementation and write your own.

Have fun ;)