Extjs3.4 grid中添加一列复选框

　　var sm = new Ext.grid.CheckboxSelectionModel();

    var cm = new Ext.grid.ColumnModel(
        [
            sm,
            new Ext.grid.RowNumberer(),
            {
                header: '立案号',
                dataIndex: 'AccidentNumber',
                width: 100
            }, {
                header: '车牌号',
                dataIndex: 'VehicleNumber',
                width: 100
            }, {
                header: '出险日期',
                dataIndex: 'AccidentDate',
                width: 100
            }, {
                header: '出险地点',
                dataIndex: 'AccidentPlace',
                width: 100
            }, {
                header: '司机',
                dataIndex: 'Driver',
                width: 100
            }, {
                header: '事故经过',
                dataIndex: 'AccidentProcess',
                width: 100
            }
        ]
    );

    var grid = new Ext.grid.GridPanel({
        store: store,
        trackMouseOver: false,
        disableSelection: true,
        autoHeight: true,
        autoWidth: true,
        autoScroll: true,
        viewConfig: {
            forceFit: true
        },
        loadMask: true,
        stripeRows: true,
        columnLines: true,
        sm: sm,
        cm: cm,

        tbar: new Ext.Toolbar({
            enableOverflow: true,
            items: [searchAction, '-', editAction, '-', MergeAction, '-', AutoMergeAction]
        }),

        bbar: new Ext.PagingToolbar({
            pageSize: 20,
            store: store,
            displayInfo: true
        })
    });