title: Morphia Query by Reference ObjectID
body: <p>Here's my entity's definition:</p>
  
  <pre><code>@Entity("Comment")
  public class Comment extends BaseEntity {
  
      @Reference
      private Merchant merchant;
  
      ...
  }
  
  @Entity("Merchant")
  class Merchant extends BaseEntity{
      @Id
      @Property("id")
      protected ObjectId id;
  
      ...
  }
  </code></pre>
  
  <p>And here's my data:</p>
  
  <pre><code>comment:{
  "_id": ObjectId("546c1ac64652e5180dc21577"),
  "merchant" : DBRef("Merchant", ObjectId("546c1ac64652e5180dc21576")),
  
  ...
  }
  </code></pre>
  
  <p>When I create a Query like:</p>
  
  <pre><code>Query&lt;Comment&gt; query = ds.createQuery(Comment.class);
  query.field("merchant").equal(new ObjectId("546c1ac64652e5180dc21576"));
  
  commentDao.findOne(query);
  </code></pre>
  
  <p>There's no result returned, I'd like to ask which is the right way to query a comment data with merchant's ObjectId?</p>
  
  <p>Thanks for your help.</p>