我有一堆文件，格式如下:{_id:"5b38c85f213751038ff4d0bb",classes:[{_id:"5b38c85f21375103ytg65ju7",standard:"12",numberOfStudents:45},{_id:"5b38c85f2137510370987hgd",standard:"11",numberOfStudents:51},]}我想:按_id和特定类的_id搜索文档集合。更新该类(class)的numberOfStudents。只返回类对象。因此对于_id:"5b38c85f213751038ff4d0bb"&classes._id:"5