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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，但在投影中它被忽略了。

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 }
           ]
}

而且我要：

具有“邮政编码”的文件：“63109”

并且学生数组应仅包含“studentId”为“2”的子文档（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: []
}

但是我在“students”数组中获取了每个子文档。似乎 $elemMatch 投影被忽略了。

我正在使用的查询：

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

有没有办法做到这一点？

PS我正在使用co-monk。

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