的MongoDB（猫鼬）如何使用$ elemMatch

Question

返回所有文件中的字段MongoDB的文档（http://docs.mongodb.org/manual/reference/operator/projection/elemMatch/）根据：

{ 
_id: 1, 
school: "school A", 
zipcode: 63109, 
students: [ 
       { name: "john", school: 102, age: 10 }, 
       { name: "jess", school: 102, age: 11 }, 
       { name: "jeff", school: 108, age: 15 } 
      ] 
} 
{ 
_id: 2, 
school: "school B", 
zipcode: 63110, 
students: [ 
       { name: "ajax", school: 100, age: 7 }, 
       { name: "achilles", school: 100, age: 8 }, 
      ] 
} 

{ 
_id: 3, 
school: "school C", 
zipcode: 63109, 
students: [ 
       { name: "ajax", school: 100, age: 7 }, 
       { name: "achilles", school: 100, age: 8 }, 
      ] 
} 

{ 
_id: 4, 
school: "school D", 
zipcode: 63109, 
students: [ 
       { name: "barney", school: 102, age: 7 }, 
      ] 
} 

启动：

schools.find（{邮编：63109}，{学生：{$ elemMatch：{school：102 }}}，function（err，school）{...}

该操作返回以下文档：

{ “_id”：1， “生”：[{ “名称”： “约翰”， “学校”：102， “年龄”： 10}]} { “_id”：3} { “_id”：4， “学生”：[{ “Name”： “巴尼”， “学校”：102， “时代”：7}]}

但我需要学校的价值提交过...

{“_id”：1，“school”：“School A”，“students”：[{“name”：“john”，“school”：102，“age” ： 10}]} {“_id”：3，“school”：“School C”} {“_id”：4，“school”：“School D”，“students”：[{“name”： “barney ”， “学校”：102， “时代”：7}]}

，我无法找到一个方法来做到这一点...

Answer 1

http://docs.mongodb.org/manual/reference/method/db.collection.find/

如果投影参数被指定，匹配文件 只包含投影字段和_id字段。您可以 （可选）排除_id字段。

但是...我被迫使用的字段要返回：

schools.find({ zipcode: 63109}, {school: 1, students: { $elemMatch: { school: 102 } } }, function (err, school) { ...} 

和一切似乎正常工作...