仅获取 mongodb 集合中具有条件的第一个匹配文档
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【中文标题】仅获取 mongodb 集合中具有条件的第一个匹配文档【英文标题】：Fetching only the first matched document with condition in mongodb collection 【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】：
【参考方案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...【参考方案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】：
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