MongoDB 子文档投影

Question

需要一些基于子文档属性值的 Mongo 查询的帮助，同时还要抑制该子文档属性出现在结果中。

这是我的用户对象:

{
  "username" : "abc",
  "emails" : [
    {
      "address" : "abc1@email.com",
      "default" : true
    },
    {
      "address" : "abc2@email.com",
      "default" : false
    }
  ]
},
{
  "username" : "xyz",
  "emails" : [
    {
      "address" : "xyz1@email.com",
      "default" : false
    },
    {
      "address" : "xyz2@email.com",
      "default" : true
    }
  ]
}

我的目标是获得以下输出(带有“emails.default”:true 但结果中没有出现“emails.default”属性):

{
  "username" : "abc",
  "emails" : [
    {
      "address" : "abc1@email.com",
    }
  ]
},
{
  "username" : "xyz",
  "emails" : [
    {
      "address" : "xyz2@email.com",
    }
  ]
}

使用 $ 位置运算符:

collection.find({"emails.default":true}, {"username":1,"emails.$":1})

我得到了正确的电子邮件子文档，但我仍然得到了“emails.default”属性:

{
  "username" : "abc",
  "emails" : [
    {
      "address" : "abc1@email.com",
      "default" : true
    }
  ]
},
{
  "username" : "xyz",
  "emails" : [
    {
      "address" : "xyz2@email.com",
      "default" : true
    }
  ]
}

出于某种原因，当我使用此语句时:

collection.find({"emails.default":true}, {"username":1,"emails.address":1})

我得到以下结果(好像忽略了语句的查询部分)

{
  "username" : "abc",
  "emails" : [
    {
      "address" : "abc1@email.com",
    },
    {
      "address" : "abc2@email.com",
    }
  ]
},
{
  "username" : "xyz",
  "emails" : [
    {
      "address" : "xyz1@email.com",
    },
    {
      "address" : "xyz2@email.com",
    }
  ]
}

非常感谢，提前致谢。

Answer 1

使用位置运算符(在 MongoDB 2.6 中)，您不能有选择地排除要匹配的数组元素的字段。

但是，您可以使用 MongoDB 2.2+ 中的聚合框架来实现您想要的结果:

db.user.aggregate(

    // Match on relevant documents to optimize query
    { $match: {
        username: "abc"
    }},

    // Convert the "emails" address into a stream of documents
    { $unwind: "$emails" },

    // Only include the emails with default:true
    { $match: {
        "emails.default" : true
    }},

    // Project the desired fields
    { $project: {
        _id : 0,
        username: 1,
        "emails.address": 1
    }}
)

示例结果:

{
    "username" : "abc",
    "emails" : {
        "address" : "abc1@email.com"
    }
}