Extjs 中 下拉框 放入grid中

把下拉框放入grid中，为什么下拉框选中后的显示值 是value值，还有批量保存grid时怎么能取到combobox的VALUE值，新手，请各位大神指点,下面附上部分代码
    
     { header: '单位名称', dataIndex: 'unitname',
                    field:
                 { xtype: 'combo',
                     editable: false,
                     emptyText: "请选择...",
                     valueField: "id",
                     displayField: "text",
                     mode: 'local',
                     store: storeddl
                 }
                }            var storeddl = Ext.create('Ext.data.Store', {
                proxy: {
                    type: 'ajax',
                    url: "../../Handler/SystemHandler/UnitHandler.ashx?Lx=ddl"
                },
                root: 'items',
                forceSelection: true,
              //  lazyRender: true,
                fields: ['id', 'text'],
                listeners:
         {
            select: selectRow,
            focus: eventFocus
         }
            });extjs