extjs4.1 grid嵌入combobox,選中后顯示displayField而不是valueField

var isEdit = false;//這變量和后面的方法寫在ext.onready里面

    cbstore = createStore();
    function rendererData(value, metadata, record) {
        if (isEdit) {
            var index = cbstore.find(Ext.getCmp('cengm').valueField, value);
            var record = cbstore.getAt(index);
            return record.data.name;
        } else {
            return value;
        }

    }

function createStore() {
    // The data store holding the states; shared by each of the ComboBox examples below
    return Ext.create('Ext.data.Store', {
        autoDestroy: true,
        model: 'State',
        data: states
    });
}

{
                    header: '層面',
                    width: 70,
                    sortable: true,
                    dataIndex: 'cengm',
                    field: {
                        xtype: 'combobox',
                        id: 'cengm',
                        typeAhead: true,
                        triggerAction: 'all',
                        queryMode: 'local',
                        selectOnTab: true,
                        lazyRender: true,
                        displayField: 'name',
                        valueField: 'id',
                        store: createStore(),
                        listeners: {
                            select: function (combo, record, index) {
                                isEdit = true;
                            }//選中后事件,其他事件沒試驗成功
                        }
                        //value: '01'
                        //editable: false
                    },
                    renderer: rendererData
                }