MongoDB 多对多的关系应该怎样描述？

可以这样设计，collection里面每个文档存储一个学生的选课数据：

// students
{
    _id: ObjectId('4e7020cb7cac81af7136236b'),
    name: "...",
    choose_lesson: [
        {_id:ObjectId('4e7020cb7cac81af71362361'),
         lesson_name: "..."
        },
        {_id:ObjectId('4e7020cb7cac81af71362362'),
         lesson_name: "..."
        }
    ]
}

学生选了哪些课？

db.students.find(
    {_id: ObjectId('4e7020cb7cac81af7136236b')},
    {choose_lesson: 1}
)

一门课有哪些学生选择？

db.students.find(
    {choose_lesson: {$elemMatch: {lesson_name: "..."}}},
    {name: 1}
)