title: extjs 4.1 converting a dynamic combobox into dynamic radiogroup
body: <p>After loading in a store of user roles [id, name] I can easily make the following form field which dynamically populates a drop down list of roles (User, Admin, ...)</p>
  
  <pre><code>{
      xtype: 'combobox',
      name: 'roleIds',
      queryMode: 'local',
      pinList: false,
      fieldLabel: 'Role',
      store: 'Roles',
      displayField: 'name',
      valueField: 'id',
      allowBlank: false
  }
  </code></pre>
  
  <p>This works fine but what are the proper steps required to turn this into a radiogroup?  How do I make sure the proper radio button is selected when editing a record?  How do I specify the default when the form is empty and making a new user?</p>
  
  <p>Thanks</p>