title: How to find records by foreign key (dbref) in morphia and mongodb?
body: <p>I have following entity:</p>
  
  <pre><code>class Linf {
       @Id
       ObjectId id;
       @Reference
       Denied denied;
  }
  </code></pre>
  
  <p>I want to find all Linfs that have Denied object with certain id. How can I do this? Will this query employ indexes? I want to avoid full scan if possible.</p>
  
  <p>Thanx. </p>