title: Devise redirect to sign_in_path for unauthenticated users without using :database_authenticatable
body: <p>I am using cancan, devise, and omniauth in a rails app. I'm quite happy with omniauth, and the method current_user is helpful. Ability class is also helpful. When a user registers, I only get his email from omniauth, and manually create a User for him with some parameters. So I am not using the devise way of creating users.</p>
  
  <p>Consequently, I have no need for  :database_authenticatable. Having it prevents me from saving a user without a password. Not having it however, makes users go to '/' instead of '/users/sign_in' when they hit an unauthorized resource.</p>
  
  <p>I would like to not use :database_authenticatable, and have users go to '/users/sign_in' if they are unauthorized. How would I do that?</p>
  
  <p>Note: almost all my controllers have fine-grained authenticate_user! beforefilter, for example:</p>
  
  <pre><code>class UsersController &lt; ApplicationController
  
      before_filter :authenticate_user!, :except =&gt; [:index, :index_small, :show,
       :new , :sign_in ]
  </code></pre>
  
  <p>so I'm not sure if I can do anything with <code>before_filter :authenticate_user!</code> in application_controller. If I can get rid of this filter on all the controllers, it would be nice also.</p>