EXTJS4 如何默认选中树(treeview)的节点

   var record = treePanel.getStore().getNodeById('item_id');
   treePanel.getSelectionModel().select(record)

完整的示例如下：

var store = Ext.create('Ext.data.TreeStore', {
    root: {
        expanded: true,
        children: [
            { text: "detention", leaf: true },
            { text: "homework", expanded: true, children: [
                { text: "book report", leaf: true },
                { text: "alegrbra", id:'select', leaf: true}
            ] },
            { text: "buy lottery tickets", leaf: true }
        ]
    }
});

Ext.create('Ext.tree.Panel', {
    title: 'Simple Tree',
     200,
    height: 150,
    store: store,
    rootVisible: false,
    renderTo: Ext.getBody(),
    listeners:{
        afterrender:function(){
          var record = this.getStore().getNodeById('select');
            this.getSelectionModel().select(record)
        }
    }
});

运行效果如图：树产生后，默认选中了ID='select'的这一项