title: How to pass treestore in combobox in Extjs
body: <p>I have a combobox which require a treestore as its store.
  I have tried below code but its not working.</p>
  
  <pre><code>Ext.define('DemoGroupCombo', {
      extend: 'Ext.data.TreeStore',
      fields: ['text', 'value'],
      proxy: {
          type: 'rest',
          url: '../SamplePaging/GetComboTree',
          //data: groupStoreData,
          reader: {
              type: 'json',
              rootProperty: 'children'
          }
      }
  });
  </code></pre>
  
  <p><strong>Expected json result from api:</strong></p>
  
  <pre><code>var groupStoreData =  
  //new Ext.data.TreeStore({
  //  root:
  {
      expanded: true, children: [
          {
              //checked: false, text: "All", expanded: true, children: [
              //  {
              checked: false, text: "Numbers", expanded: true, children: [
                  { checked: false, text: '1', value: '1', leaf: true },
                  { checked: true, text: '2', value: '2', leaf: true },
                  { checked: false, text: '3', value: '3', leaf: true },                  
                  {
                      checked: false, text: '4', value:'4', leaf: true
                  }
              ]
          }
      ]
      //}]
  }
  </code></pre>
  
  <p><strong>Combobox:</strong></p>
  
  <pre><code>{
      xtype: 'combobox',          
      selModel: {
         selType: 'checkboxmodel'
      },
      queryMode: 'local', 
      displayField: 'text',
      valueField: 'value',        
      store: { type: 'DemoGroupCombo' }
  }
  </code></pre>
  
  <p>Right now I am getting this error message on display:</p>
  
  <blockquote>
    <p>Error: [Ext.createByAlias] Unrecognized alias: store.DemoGroupCombo</p>
  </blockquote>