title: Sencha extjs creating and populating a Model instance via Ajax
body: <p>I have a Model defined with a mapping:</p>
  
  <pre><code>Ext.define('IM.model.Source', { extend : 'Ext.data.Model', fields : [
    {
      "name": "billref_id",
      "mapping": "billref.id"
    },...
  </code></pre>
  
  <p>If I create a Grid with a Store using this Model and load it, every record in the Store has the billref_id attribute correctly initialised to the value of billref.id from my JSON data.</p>
  
  <p>If I instead load the same data using an Ext.Ajax.request, and then feed that data into a Model instance this way:</p>
  
  <pre><code>var response = Ext.Ajax.request({
              async: false,
              method:'GET',
              url: 'im_read.json',
              params:{pkValue:1}
  });
  var items = Ext.decode(response.responseText);
  record = Ext.create('IM.model.Source', items.rows[0]);
  </code></pre>
  
  <p>the resulting record does not have the mapped fields populated from the JSON data.
  Only the non-mapped members of the model have values assigned.</p>
  
  <p>Is it a known issue that creating and populating a Model instance this way doesn't work with mapped fields, or am I doing something wrong?
  Thanks,</p>