title: Binding an ExtJs combobox form field's selected item
body: <p>I've just started working with extjs 4 and I'm trying to use it alongside asp.net MVC 3. I'm having trouble setting up a typical form for editing a record where one of the fields uses a ComboBox. I cant seem to find an elegant way to bind the initial selected value to the value of a property on my model. Does anyone have any suggestions. I would think this should be simple to accomplish. Here's my form so far... Thanks in advance.</p>
  
  <pre><code>          {
              xtype: 'form',
              items: [
                  {
                      xtype: 'combobox',
                      fieldLabel: 'Group',
                      displayField: 'Name',
                      valueField: 'GroupId',
                      store: 'GroupStore',
                      allowBlank: false,
                      forceSelection: true,
                      editable: false,
                      queryMode: 'local'                       
                  },
                  {
                      xtype: 'textfield',
                      name: 'Description',
                      fieldLabel: 'Description'
                  }
              ]
          }
  </code></pre>