title: Objectify: Filter by Ref (or Key) of a Certain Kind
body: <p>Say I have these classes:</p>
  
  <pre><code>@Entity class MyEntity {
      @Id String id;
      @Index Ref&lt;?&gt; ref;
  }
  
  @Entity class Kind2 {
      ...
  }
  </code></pre>
  
  <p>Can I query for all <code>MyEntitiy</code> objects where <code>ref</code> refers to any instance of <code>Kind2</code>?  If so, how?</p>