关于mongoose的关联查询，举例说明：

1. 初始化3个Schema和3个model

var studentSchema = new Schema({
    name  : String,
    age   : String,
    school: {
        type: Schema.Types.ObjectId,
        ref : 'school'
    }
});
var schoolSchema = new Schema({
    name    : String,
    students: [
        {
            type: Schema.Types.ObjectId,
            ref : 'student'
        }
    ],
    city    : {
        type: Schema.Types.ObjectId,
        ref : 'city'
    }
});
var citySchema = new Schema({
    name  : String,
    school: [
        {
            type: Schema.Types.ObjectId,
            ref : 'school'
        }
    ]
});
var Student = mongoose.model('student', studentSchema);
var School = mongoose.model("school", schoolSchema);
var City = mongoose.model("city", citySchema);

2. 初始化数据

var city = new City({
    name  : '北京'，
    school: []
});

city.save(function (err, city) {
    var school = new School({
        name    : 'Test',
        students: [],
        city    : city._id
    });
    school.save(function (err, school) {
        var student = new Student({
            name  : 'Tom',
            age   : 20,
            school: school._id
        });
        student.save();
    });
});

3. 进行关联查询，进行查询的时候要把初始化的数据注释掉

Student.findOne({name: 'Tom'})
       .populate({
           path: 'school',
           populate: {path: 'city'}
       })
       .exec(function (err, data) {
          console.log(data.name + ' 所在学校为：' + data.school.name + "，所在城市为：" + data.school.city.name);
})

4. 结果

Tom 所在学校为：Test，所在城市为：北京