title: Extjs Model Fields with Subfields
body: <p>Anyone know how you model subfields of any field in ExtJS? For example</p>
  
  <p>Ext.data.Model:</p>
  
  <pre><code>fields:[
     {name: 'id', type: 'int'},
     {name: 'title', type: 'string'},
     {name: 'description', type: 'string'},
     {name: 'priority', type: 'auto', fields:[
        {name: 'code', type: 'string'}
     ]},
     {name: 'createdBy', type: 'auto'},
  ]
  </code></pre>
  
  <p>then in my grid panel</p>
  
  <p>Ext.grid.Panel</p>
  
  <pre><code>columns:[
      {header:'Title', dataIndex:'title', flex:1},
      {header:'Priority', dataIndex:'code'}
  ],
  </code></pre>
  
  <p>Any idea how I access the dataIndex 'code' under 'priority'? thanks in advance!</p>