title: Extjs 4 combobox default value
body: <p>I'm migrating my application from ExtJs 3 to 4 version.
  I have several comboboxes at my formPanel, and previously I've used hiddenName
  and all that <a href="http://www.sencha.com/learn/Ext_FAQ_ComboBox" rel="noreferrer">stuff</a> to submit valueField instead of displayField.</p>
  
  <p>All my adaptation works fine (value field IS submitting), but I can't set the default values for comboboxes, they are shown as empty after page load.
  Previously, I did that just with specifying the 'value' parameter in config.
  Is there any ideas how to fix that?</p>
  
  <p>My code - Model and Store:</p>
  
  <pre><code>Ext.define('idNamePair', {
      extend: 'Ext.data.Model',
      fields: [
          {name: 'id', type: 'string'},
          {name: 'name',  type: 'string'}
      ]
  });
  
  var dirValuesStore = new Ext.data.Store({
      model: 'idNamePair',
      proxy: {
          type: 'ajax',
          url: '../filtervalues.json',
          reader: {
              type: 'json',
              root: 'dir'
          }
      },
      autoLoad: true
  });
  </code></pre>
  
  <p>Combo config:</p>
  
  <pre><code>{
      triggerAction: 'all',
      id: 'dir_id',
      fieldLabel: 'Direction',
      queryMode: 'local',
      editable: false,
      xtype: 'combo',
      store : dirValuesStore,
      displayField:'name',
      valueField:'id',
      value: 'all',
      width: 250,
      forceSelection:true
  }
  </code></pre>