title: rails mongoid criteria find by association
body: <p>I'm trying to find a record by associated username which is included in a belongs_to relation, but it's not working.</p>
  
  <p>Articles belong to Users
  Users have many articles</p>
  
  <p><code>Article.where(user_id: someid)</code> works fine, but I'd like to use the username as reference which is stored in the Users table.</p>
  
  <pre><code>Article.includes(:user).where(:username =&gt; "erebus")
  Article.includes(:user).where("user.username" =&gt; "erebus")
  </code></pre>
  
  <p>I also have <code>identity_map_enabled: true</code></p>
  
  <p><code>Article.includes(:user).inclusions</code> returns the relation details</p>
  
  <p>Doesn't work, what am I not understanding?</p>