MongoDB $ elemMatch投影忽略了

Question

我正在使用Node.js，Koa，MongoDB，Monk。

当我在查询中使用$ elemMatch时，它工作正常，但在投影中它会被忽略。

我有这样的数据

{
 _id: 1,
 zipcode: "63109",
 students: [
              { studentId: "1", name: "john", school: 102, age: 10 },
              { studentId: "2", name: "jess", school: 102, age: 11 },
              { studentId: "3", name: "jeff", school: 108, age: 15 }
           ]
}
{
 _id: 2,
 zipcode: "63110",
 students: [
              { studentId: "1", name: "john", school: 102, age: 10 },
              { studentId: "4", name: "achilles", school: 100, age: 8}
           ]
}
{
 _id: 3,
 zipcode: "63109",
 students: []
}
{
 _id: 4,
 zipcode: "63110",
 students: [
              { studentId: "3", name: "jeff", school: 108, age: 15 }
           ]
}

我想要：

具有＆＃34;邮政编码的文件＆＃34; ：＆＃34; 63109＆＃34;

并且学生数组应该只包含那个＆＃39; studentId＆＃39;是＆＃39; 2＆＃39; （如果存在）。

即。我想要这个：

{
 _id: 1,
 zipcode: "63109",
 students: [
              { studentId: "2", name: "jess", school: 102, age: 11 }
           ]
}
{
 _id: 3,
 zipcode: "63109",
 students: []
}

但是我得到了＃34;学生的所有子文档＆＃34;阵列。好像$ elemMatch投影被忽略了。

我正在使用的查询：

collection.find({"zipcode":"63109"}, {students:{$elemMatch:{studentId:"2"}}})

有没有办法实现这个目标？

P.S。我正在使用合作僧侣。