title: How to use a concern in a Rails initializer?
body: <p>In my Ruby on Rails project I have a concern <code>app/controllers/concerns/common.rb</code>:</p>
  
  <pre><code>module Common
    extend ActiveSupport::Concern
  
    private
  
    def foo
    end
  
  end
  </code></pre>
  
  <p>I want to use its <code>foo</code> function inside <code>config/initializers/devise.rb</code>:</p>
  
  <pre><code>Devise.setup do |config|
    ...
    foo
    ...
  end
  </code></pre>
  
  <p>For now I failed to somehow load <code>common.rb</code> inside <code>config/initializers/devise.rb</code> - how to do it?</p>
  
  <p>Currently I'm using a workaround - I define the <code>foo</code> function inside <code>config/application.rb</code> and it works, but it breaks the DRY principles.</p>