treegrid中右键菜单如何拿到被点击列的field是哪个？

felt

menu的beforeopen事件内处理
onbeforeopen="onBeforeOpen"
function onBeforeOpen(e){

      var cellId=e.htmlEvent.target.parentNode.id;
      var columnId = cellId.split("$cell$")[1];
       var columns = grid.getBottomColumns();
       var column = columns[columnId-1];
      alert(column.field)
}