ext如何读取嵌套的子json节点数据

{
    "rows": [
        {
            "userName": "system",
            "userInfo": {
                "active": true,
                "address": "深圳市",
                "birthday": "2014-11-03"
            }
        }
    ]
}

    Ext.onReady(function () {
        Ext.define('nestedModel', {
            extend: 'Ext.data.Model',
            fields: ["userName",
                { "name": "active", mapping: "userInfo.active" },
                { "name": "address", mapping: "userInfo.address" },
                { "name": "birthday", mapping: "userInfo.birthday" }]
        });
        var store = Ext.create('Ext.data.Store', {
            model: 'nestedModel',
            proxy: { url: 'nesteddata.json', type: 'ajax', reader: { root: 'rows', type: 'json' } },
            listeners: {
                load: function () {
                    alert(this.getAt(0).get('active'))//true
                    alert(this.getAt(0).get('address'))//深圳市
                }
            }
        });
        store.load();
    });