title: Mapping Mongo id field with Google Gson
body: <p>I'm reading a Document from MongoDB which contains its unique identifier:</p>
  
  <pre><code>{
      "_id" : ObjectId("5526888bd3d56a86cea8ea12"),
      "name" : "user1"
  }
  </code></pre>
  
  <p>I'd like to map that with a java class</p>
  
  <pre><code>public class Mapper {
      Object _id;
      String name;
  }
  </code></pre>
  
  <p>As a result of my fromJson execution:</p>
  
  <pre><code>Mapper m = gson.fromJson(string, Mapper.class);
  </code></pre>
  
  <p>...the value stored in the _id field is <code>{$oid=5526888bd3d56a86cea8ea12}</code>. I'd like to store the id String in it. (e.g. <code>"5526888bd3d56a86cea8ea12"</code>)</p>
  
  <p>Can Gson do it for me automatically ? </p>
  
  <p>Thanks</p>