How to Return Validation Errors as JSON in the Play! Framework

How can I return validation errors in JSON format when using the Play! framework?

// Example of returning validation errors as JSON in Play!
case class UserInput(name: String, age: Int)

def validateUserInput(input: UserInput): Either[List[String], UserInput] = {
  val errors = List(
    if (input.name.isEmpty) Some("Name cannot be empty") else None,
    if (input.age < 0) Some("Age must be non-negative") else None
  ).flatten

  if (errors.isEmpty) Right(input) else Left(errors)
}

def saveUser(): Action[JsValue] = Action(parse.json) { request =>
  request.body.validate[UserInput].fold(
    _ => BadRequest(Json.obj("error" -> "Invalid input")),
    userInput => validateUserInput(userInput) match {
      case Left(errors) => BadRequest(Json.obj("errors" -> errors))
      case Right(validInput) => Ok(Json.obj("message" -> "User saved successfully"))
    }
  )
}

In the Play! Framework, you can easily return validation errors in a structured JSON format by utilizing the framework's built-in JSON handling capabilities. This approach allows you to provide clear feedback to the client regarding the validation status of the submitted data.

val errorsJson = Json.obj("errors" -> errors)
BadRequest(errorsJson)

Causes

• Improper input data received from the user.
• Validation logic fails due to incorrect field values.

Solutions

• Use the Play! framework's JSON validation support.
• Implement error handling in your API controller to capture and format validation errors.

Mistake: Failing to validate input correctly before processing.

Solution: Ensure all inputs are validated against your specified rules before further operations.

Mistake: Not formatting the error response in a user-friendly JSON structure.

Solution: Structure the JSON properly using key-value pairs to enhance clarity.