title: play morphia find a Reference object by ObjectId
body: <p>I have the following class "Ground":</p>
  
  <pre><code>@Entity("grounds")
  public class Ground {
  
  @Id
  public String id;
  
  public String name;
  public String password;
  
  @Reference
  List&lt;User&gt; members = new ArrayList&lt;User&gt;();
  </code></pre>
  
  <p>I created a new Ground and added one member as <code>@Reference</code>. When I try to find the ground by the member <code>@Id</code> I just get an empty query. Here's my code:</p>
  
  <pre><code>    Datastore ds = Database.getDatastore();
      User member = ds.get(User.class, new ObjectId(id));
  
      Query&lt;Ground&gt; query = ds.createQuery(Ground.class).disableValidation()
              .filter("members", member);
  </code></pre>