custom_failure.rb

class CustomFailure < Devise::FailureApp
  
  # Never do http authentication through Devise
  def http_auth?
    false
  end

  def redirect_url
    send(:"new_#{scope}_session_path", :format => (request.xhr? ? 'js' : nil ))
  end
  
end

devise.rb

Devise.setup do |config|
  
  ...

  # ==> Warden configuration
  # If you want to use other strategies, that are not (yet) supported by Devise,
  # you can configure them inside the config.warden block.
  config.warden do |manager|
    manager.failure_app = CustomFailure
  end
end

routes.rb

MyApp::Application.routes.draw do

  # Devise authentication
  devise_for :users,
    :controllers => { :sessions => '/sessions', :registrations => '/registrations' }
  end

  ...

end

sessions_controller.rb

class SessionsController < Devise::SessionsController
  def sign_in_and_redirect(resource_or_scope, resource=nil)
    scope      = Devise::Mapping.find_scope!(resource_or_scope)
    resource ||= resource_or_scope
    redirect_url = stored_location_for(scope)
    respond_to do |format|
      format.js do
        sign_in(scope, resource) unless warden.user(scope) == resource
        if redirect_url.present?
          redirect_url = "#{redirect_url}.js" unless redirect_url[-3..-1] == '.js'
          redirect_url += redirect_url.match(/\?/) ? '&' : '?'
          redirect_url += "after_authentication=true"
          redirect_to redirect_url
        else
          render(:update) do |page|
            page << render('/shared/after_authentication')
          end
        end
      end
      format.html { super }
    end
  end
end