title: ExtJS 4.1: how to set a preselected item in a combo box?
body: <p>I'm working with ExtJS 4.1, I need to create a combo box containing a list of customers and I'd like to set a specific pre-selected item in it, but I don't know how to do it. 
  Here's the code to create my combo box:</p>
  
  <pre><code>xtype: 'combobox',
  fieldLabel: 'customer',
  name: 'customer_id',
  allowBlank:false,
  afterLabelTextTpl: required,
  //data store
  store: Ext.create('Ext.data.Store', {
      autoDestroy: true,
      model: 'customer_model',
      autoLoad: true,
      proxy: {
          type: 'ajax',
      url: 'load.php?item=customer',            
      reader: {
          type: 'json',
          successProperty: 'success',
          root: 'data'
      }
      }
  }),
  valueField: 'id',
  displayField: 'company',
  typeAhead: true,
  queryMode: 'remote',
  emptyText: ''
  </code></pre>
  
  <p>As you can see my combo box is filled by a data store, that data store is built on a data model called 'customer_model'. Here's the code for data model:</p>
  
  <pre><code>Ext.define('customer_model', {
      extend: 'Ext.data.Model',
      fields: [
          {type: 'int', name: 'id'},
          {type: 'string', name: 'company'},
          {type: 'string', name: 'vat'},
          {type: 'string', name: 'ssn'},
          {type: 'int', name: 'ref_id'}
      ]
  }); 
  </code></pre>
  
  <p>Well, I'd like to configure my combo box so that a certain item, for instance the customer having id equals to 1, is automatically selected when the page is loaded.
  Can anyone help me ?
  Thanks in advance.
  Enrico.              </p>