Custom Devise 401 unauthorized response

I'm working on a JSON-based API for my Rails 3.1 app. I'd like to provide a custom failure response instead of the default, which is:

{"error":"You need to sign in or sign up before continuing."}

My API controller includes a before_filter call to authenticate_user!, which is what is rendering this JSON response.

While searching, I came across this StackOverflow question, which references this Devise wiki entry. Unfortunately, the wiki entry isn't verbose enough for me to understand what it's telling me. Specifically, I have no clue where I'm supposed to put that code such that Devise/Warden knows to render what I want returned.

From the comments on the other SA question, it sounds like I don't need to call custom_failure! since I'm us开发者_运维知识库ing a version of Devise above 1.2 (1.4.2 to be specific). However, the wiki entry doesn't explain where the render call should go such that authenticate_user! knows to use that instead of its own render call.

Where does this render call go?

Edit: I'm not just trying to change the message itself (a la the devise en.yml config); I'm trying to change the actual format of the response. Specifically, I want to return this:

render :text => "You must be logged in to do that.", :status => :unauthorized

For reference in case anyone else stumbles upon this question when looking for how to customize the json error response when a failed login attempt is made using Devise, the key is to use your own custom FailureApp implementation. (You can also use this approach to override some redirect behavior.)

class CustomFailureApp < Devise::FailureApp
  def respond
    if request.format == :json
      json_error_response
    else
      super
    end
  end

  def json_error_response
    self.status = 401
    self.content_type = "application/json"
    self.response_body = [ { message: i18n_message } ].to_json
  end
end

and in your devise.rb, look for the config.warden section:

  config.warden do |manager|
    manager.failure_app = CustomFailureApp
  end

Some related info:

At first I thought I would have to override Devise::SessionsController, possibly using the recall option passed to warden.authenticate!, but as mentioned here, "recall is not invoked for API requests, only for navigational ones. If you want to customise the http status code, you will have better luck doing so at the failure app level."

Also https://github.com/plataformatec/devise/wiki/How-To%3a-Redirect-to-a-specific-page-when-the-user-can-not-be-authenticated shows something very similar for redirection.

If you're simply wanting to change the text displayed with the error message, I believe you can just edit the locale file (/config/locales/devise.en.yml).

The RailsCast on this topic might be helpful too, if you want more specific details. You can find it at http://railscasts.com/episodes/210-customizing-devise