仅获取 mongodb 集合中具有条件的第一个匹配文档答案

【问题标题】：Fetching only the first matched document with condition in mongodb collection仅获取 mongodb 集合中具有条件的第一个匹配文档
【发布时间】：2017-05-09 02:54:53
【问题描述】：

这是我的收藏

[
    {
        _id: "585be0da13385513689f704a",
        site: "NBT",
        url: "m.nbt.com",
        label: "NBT Home",
        platform: "mobile",
        speed: 61,
        date: "2016-12-22T14:18:57.975Z"
    },
    {
        _id: "585be0da13385513689f704b",
        site: "NBT",
        url: "nbt.com",
        label: "NBT Home",
        platform: "desktop",
        speed: 75,
        date: "2016-12-22T14:15:57.975Z"
    },
    {
        _id: "585be0da13385513689f704c",
        site: "MT",
        url: "m.mt.com",
        label: "MT Home",
        platform: "mobile",
        speed: 40,
        date: "2016-12-22T14:01:57.975Z"
    },
    {
        _id: "585be0da13385513689f704d",
        site: "NBT",
        url: "m.nbt.com",
        label: "NBT Home",
        platform: "mobile",
        speed: 90,
        date: "2016-12-22T12:18:57.975Z"
    }
]

预期输出：

[
    {
        _id: "585be0da13385513689f704b",
        site: "NBT",
        url: "nbt.com",
        label: "NBT Home",
        platform: "desktop",
        speed: 75,
        date: "2016-12-22T14:15:57.975Z"
    },
    {
        _id: "585be0da13385513689f704c",
        site: "MT",
        url: "m.mt.com",
        label: "MT Home",
        platform: "mobile",
        speed: 40,
        date: "2016-12-22T14:01:57.975Z"
    },
    {
        _id: "585be0da13385513689f704d",
        site: "NBT",
        url: "m.nbt.com",
        label: "NBT Home",
        platform: "mobile",
        speed: 90,
        date: "2016-12-22T12:18:57.975Z"
    }
]

基本上我想要第一个匹配的不同（标签+平台）组合文档。

正如您在 colloection 中看到的那样，标签-NBT Home 存在于 3 个文档中，但我只想检索标签 + 平台的唯一组合。所以 NBT Home for mobile 和 NBT Home for desktop 是我所期待的。

请帮忙。

【问题讨论】：

标签： javascript node.js mongodb mongoose schema

【解决方案1】：

试试这个：

db.collection.aggregate([
   {
      $group:{
         _id:{
            label:"$label",
            platform:"$platform"
         },
         site:{
            $first:"$site"
         },
         url:{
            $first:"$url"
         },
         speed:{
            $first:"$speed"
         }
      }
   }
])

输出：

{
    "_id" : {
        "label" : "MT Home",
        "platform" : "mobile"
    },
    "site" : "MT",
    "url" : "m.mt.com",
    "speed" : 40
}
{
    "_id" : {
        "label" : "NBT Home",
        "platform" : "desktop"
    },
    "site" : "NBT",
    "url" : "nbt.com",
    "speed" : 75
}
{
    "_id" : {
        "label" : "NBT Home",
        "platform" : "mobile"
    },
    "site" : "NBT",
    "url" : "m.nbt.com",
    "speed" : 61
}

【讨论】：

这仅返回 2 条记录，但预期输出为 3。
MT Home 和 NBT Home+mobile 返回。 NBT Home + 桌面不见了。
@MohitRathi 再看看我发布的输出，返回了 3 个文档，并且存在 NBT 家庭 + 桌面。这是对您提供的示例文档进行聚合查询的结果...

【解决方案2】：

在聚合管道中使用$group，然后使用$first或$limit

以下查询是使用 $first 编写的。

db.collection.aggregate([{$group:{ "_id":{"label":"$label", "platform":"$platform"}, "site": {"$first":"$site"}, "url": {"$first":"$url"}, "speed":{"$first":"$speed"}, "date":{"$first":"$date"} } }])

对于具有以下记录的样本集合

db.collection.find()


{ "_id" : "585be0da13385513689f704a", "site" : "NBT", "url" : "m.nbt.com", "labe
l" : "NBT Home", "platform" : "mobile", "speed" : 61, "date" : "2016-12-22T14:18
:57.975Z" }
{ "_id" : "585be0da13385513689f704b", "site" : "NBT", "url" : "nbt.com", "label"
 : "NBT Home", "platform" : "desktop", "speed" : 75, "date" : "2016-12-22T14:15:
57.975Z" }
{ "_id" : "585be0da13385513689f704c", "site" : "MT", "url" : "m.mt.com", "label"
 : "MT Home", "platform" : "mobile", "speed" : 40, "date" : "2016-12-22T14:01:57
.975Z" }
{ "_id" : "585be0da13385513689f704d", "site" : "NBT", "url" : "m.nbt.com", "labe
l" : "NBT Home", "platform" : "mobile", "speed" : 90, "date" : "2016-12-22T12:18
:57.975Z" }

上面的查询将给出结果

{
  "_id": {
    "label": "MT Home",
    "platform": "mobile"
  },
  "site": "MT",
  "url": "m.mt.com",
  "speed": 40,
  "date": "2016-12-22T14:01:57.975Z"
}
{
  "_id": {
    "label": "NBT Home",
    "platform": "mobile"
  },
  "site": "NBT",
  "url": "m.nbt.com",
  "speed": 61,
  "date": "2016-12-22T14:18:57.975Z"
}
{
  "_id": {
    "label": "NBT Home",
    "platform": "desktop"
  },
  "site": "NBT",
  "url ": "nbt.com",
  "speed": 75,
  "date": "2016-12-22T14:15:57.975Z"
}

【讨论】：