Using Exceptions to manage control flow in Rails Controllers

Ah yes, the Rails Controller, a source of much contention among Rails developers. So many different ways to manage control flow, load objects, respond in standard and erroneous ways. My opinion up until recently was "I'll just put a bunch of conditionals in there for different situations."

Recently, I've been working more on API endpoints and so responding with nice error messages has been more of a priority. I started using Exceptions more throughout my code thanks to Avdi Grimm, and I recently wrote and action that I'm particularly proud of. Check it out:

# This controller's job is to exchange twitter credentials for Shortmail credentials
class TwitterReverseAuthController < ApplicationController
  # First, let's make our own subclass of RuntimeError
  class Error < RuntimeError; end

  def api_key_exchange
    # Here are our required parameters. If any are missing we raise an error
    screen_name = params.fetch(:screen_name) { raise Error.new('screen_name required')  }
    token =       params.fetch(:oauth_token) { raise Error.new('oauth_token required')  }
    secret =      params.fetch(:oauth_secret){ raise Error.new('oauth_secret required') }

    # OK now let's authenticate that user. If we can't find a valid user, raise an error
    @user = User.by_screen_name(screen_name).where(
      :oauth_token => token,
      :oauth_secret => secret
    ).first or raise Error.new('user not found')

    # Now we'll build a device. I'm not catching an exception on create! here because
    # It should never fail. (I.e. a failure is actually a 500 because we don't expect it)
    @device = Device.find_or_create_by_token!(
      params.slice(:token, :description).merge(:user_id => @user.id)
    )

    render :json => { :api_key => @device.api_key }

  # Now I can simply catch any of my custom exceptions here
  rescue Error => e
    # And render their message back to the user
    render :json => { :error => e.message }, :status => :unprocessable_entity
  end
end

Here are the things I really like about this solution:

• The happy path is really clear because there's no if/else branching
• Errors are really obvious because I'm raising an exception (as opposed to "else render json that complains" which looks like a render which is not immediately apparent as a failure)
• It's super easy to handle the errors in the same way. Instead of repeating the json render with a different message all throughout the method (i.e. it's DRY)

How's this look to you? How do you organize controller control flow?