title: I'm studying Morphia, but confused with the DBRef
body: <p><strong>1</strong></p>
  
  <pre><code>@Entity
  public class Blog {
     @Id ObjectId id;
     @reference User author;
     String content;
  }
  </code></pre>
  
  <p>or</p>
  
  <p><strong>2</strong></p>
  
  <pre><code>@Entity
  public class Blog {
     @Id ObjectId id;
     ObjectId authorId;
     String content;
  }
  </code></pre>
  
  <p>which one should I choose? </p>
  
  <p>eveytime blogDAO.get(id); the first one every query would load all the User data,
  can that be very slow or waste time? </p>