title: Avoid syncing certain parts of a model
body: <p>Let's say I have a Sencha Touch model like this:</p>
  
  <pre><code>Ext.regModel('User', {
      'fields': [{
          'name': 'first_name',
          'type': 'string'
       }, {
          'name': 'last_name',
          'type': 'string'
       }, {
          'name': 'full_name',
          'type': 'string',
          'convert': function(v, record) {
              return record.data.first_name + ' ' + record.data.last_name;
          }
       }, {
          'name': 'age',
          'type': 'integer'
       }
  ]});
  </code></pre>
  
  <p>The server returns the 'first_name', 'last_name', and 'age' fields when getting records. However, when I update a user's first and/or last name, and I call <code>sync()</code> on the store, it will send all fields to the server, including full_name and age, even though I don't want that.</p>
  
  <p>I know I could just ignore that data on the server, but in some cases (with lots of fields with 'convert' for example) it adds lots of unnecessary overhead to the payload.</p>