例如树结构为;[{id:1,childrenIdList:[2,3]},{id:2,childrenIdList:[4,5]},{id:3,childrenIdList:[]},{id:4,childrenIdList:[6,7]},{id:5,childrenIdList:[]},{id:6,childrenIdList:[]},{id:7,childrenIdList:[]}]这就像;1234567如何从叶Node(id=7)开始跟踪树到根Node(id=1)？找到id=7的父级很容易；db.document.find({childrenList:{$in:[7]}},{id:1}