title: Rails Gem with model-relationships for Rails Engines
body: <p>I'm just developing a Rails application which will have many Engines.
  However, I'm not able to edit relationships inside the Engines.</p>
  
  <p>To solve this issue, I want to create a relationships-Gem which will be included in the Application and defines the relationships (see: <a href="https://stackoverflow.com/a/11835899/603126">https://stackoverflow.com/a/11835899/603126</a>).</p>
  
  <p>Let's assume, I have a User (namespaced and isolated) Engine and a Comment (namespaced and isolated) Engine.
  What I want is to override / extend the relationships inside the relationships-Gem which will share the relationships.</p>
  
  <p>So I added a file /app/models/comment.rb with these lines (to the relationships-Gem):</p>
  
  <pre><code>class Comment &lt; CommentEngine::Comment
    belongs_to :user
  end
  
  class User &lt; UserEngine::User
    has_many :comments
  end
  </code></pre>
  
  <p>If I run my rails application, the relationships won't be established.</p>
  
  <p>What am I missing?
  How can this be achieved?</p>
  
  <p>Thank you very much in advance</p>