title: Isolated engine (doorkeeper) - use helper methods from the main_app
body: <p>I want my doorkeeper views to use the application layout:</p>
  
  <p><a href="https://github.com/applicake/doorkeeper/wiki/Customizing-views" rel="nofollow">https://github.com/applicake/doorkeeper/wiki/Customizing-views</a></p>
  
  <p>This contains routes and helper methods from the main application.</p>
  
  <p>For the routes, I can prefix main_app to the path but for the helper method I get the following error:</p>
  
  <pre><code>undefined method `is_active?' for #&lt;ActionDispatch::Routing::RoutesProxy:0xade808c&gt;
  
  &lt;li class="&lt;%= main_app.is_active?("high_voltage/pages", "api") %&gt;"&gt;&lt;%= link_to t('developers'), page_path('api') %&gt;&lt;/li&gt;
  </code></pre>
  
  <p>Why is this? The helper is in <code>app/helpers/application_helper.rb</code></p>