Monkey patching Devise (or any Rails gem)

Question

I'm using the Devise authentication gem in my Rails project, and I want to change the keys it's using in flash alerts. (Devise uses :notice and :alert flash keys, but I want to change them to :success and :error so that I can display nice green/red boxes with Bootstrap.)

So I want to be able to somehow override the set_flash_message method in DeviseController.

Here's the new method:

def set_flash_message(key, kind, options = {})

  if key == 'alert'
    key = 'error'
  elsif key == 'notice'
    key = 'success'
  end

  message = find_message(kind, options)
  flash[key] = message if message.present?

end

But I just don't know where to put it.

---

UPDATE:

Based on an answer I created a config/initializers/overrides.rb file with the following code:

class DeviseController
    def set_flash_message(key, kind, options = {})
       if key == 'alert'
          key = 'error'
       elsif key == 'notice'
          key = 'success'
       end
       message = find_message(kind, options)
       flash[key] = message if message.present?
    end
end

But this causes an error on every Devise action:

Routing Error: undefined method 'prepend_before_filter' for Devise::SessionsController:Class

Answer 1

If you try to reopen a class, it's the same syntax as declaring a new class:

class DeviseController
end

If this code is executed before the real class declaration, it inherits from Object instead of extending the class declared by Devise. Instead I try to use the following

DeviseController.class_eval do
  # Your new methods here
end

This way, you'll get an error if DeviseController has not been declared. As a result, you'll probably end up with

require 'devise/app/controllers/devise_controller'

DeviseController.class_eval do
  # Your new methods here
end

Answer 2

Using Rails 4 @aceofspades answer didn't work for me.

I kept getting require': cannot load such file -- devise/app/controllers/devise_controller (LoadError)

Instead of screwing around with load order of initializers I used the to_prepare event hook without a require statement. It ensures that the monkey patching happens before the first request. This effect is similar to after_initialize hook, but ensures that monkey patching is reapplied in development mode after a reload (in prod mode the result is identical).

Rails.application.config.to_prepare do
  DeviseController.class_eval do
    # Your new methods here
  end
end

N.B. the rails documentation on to_prepare is still incorrect: See this Github issue