Before_filter authentication function

I am developing an API using Rails 3. I have done my own authentication logic and I use an before_filter called authenticate_user! to check if a user is logged in.

In this function I check if there is a current user session active. If there is I do nothing, but if there are no active session I want to stop the rest of the functions/resources from being used and then return an error to the calling app that the user is not logged in.

My questions are:

1. How can I "stop" the request in my authentication function if there is not current session. I do not want to redirect the user, only display an erro开发者_StackOverflow中文版r message.

2. How can I display an error message that will be sent back to the calling app? It would be best if I could "answer" in JSON (and XML if possible).

This is my current code.

def authenticate_user!

        unless current_user

            if params[:token]

                user = User.authenticate_token(params[:token])

                if user

                    session[:user_id] = user.id

                else

                    SEND BACK SOMETHING HERE AND STOP THE PROCESS?

                end

            end

        end

    end

Thankful for all input!

I also wanted to know the answer to this question.

In my case I did the following in the above else clause:

  else
      redirect_to :root, :status => 401
      return false
  end

This will return a 401 not authorized and redirect to the root and stop the filter