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" -> Json.obj("m" -> 7))
res0: play.api.libs.json.JsObject = {"k":{"m":7}}
scala> (res0 \ "p").as[String]
play.api.libs.json.JsResultException: JsResultException(errors:List((,List(ValidationError(validate.error.expected.jsstring,WrappedArray())))))
at play.api.libs.json.JsValue$$anonfun$2.apply(JsValue.scala:67)
at play.api.libs.json.JsValue$$anonfun$2.apply(JsValue.scala:67)
at play.api.libs.json.JsResult$class.fold(JsResult.scala:69)
at play.api.libs.json.JsError.fold(JsResult.scala:10)
at play.api.libs.json.JsValue$class.as(JsValue.scala:65)
at play.api.libs.json.JsUndefined.as(JsValue.scala:98)
at .<init>(<console>:12)
at .<clinit>(<console>)
at .<init>(<console>:7)
// gazillion lines more
scala> (res0 \ "k" \ "m").as[String]
play.api.libs.json.JsResultException: JsResultException(errors:List((,List(ValidationError(validate.error.expected.jsstring,WrappedArray())))))
at play.api.libs.json.JsValue$$anonfun$2.apply(JsValue.scala:67)
at play.api.libs.json.JsValue$$anonfun$2.apply(JsValue.scala:67)
at play.api.libs.json.JsResult$class.fold(JsResult.scala:69)
at play.api.libs.json.JsError.fold(JsResult.scala:10)
at play.api.libs.json.JsValue$class.as(JsValue.scala:65)
at play.api.libs.json.JsNumber.as(JsValue.scala:108)
at .<init>(<console>:12)
at .<clinit>(<console>)
at .<init>(<console>:7)
// gazillion lines more
Is there a way to get better error messages out of this API? For example, the error messages for above two cases could look like No value found at JsPath \ "p"
, Value found at JsPath \ "k" \ "m" cannot be read as type String
etc.
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 ;)
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