mongodb Mongo 计算一组文档的每个值的出现次数
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Mongo count occurrences of each value for a set of documents
提问by ritmatter
I have some documents like this:
我有一些这样的文件：
{
"user": '1'
},
{ "user": '1'
},
{
"user": '2'
},
{
"user": '3'
}
I'd like to be able to get a set of all the different users and their respective counts, sorted in decreasing order. So my output would be something like this:
我希望能够获得一组所有不同的用户及其各自的计数，按降序排序。所以我的输出将是这样的：
{
'1': 2,
'2': 1,
'3': 1
}
I think this can be done with a Mongo aggregate(), but I'm having a lot of trouble figuring out the right flow for this.
我认为这可以通过 Mongo 聚合（）来完成，但是我很难找到正确的流程。
回答by Disposer
You can get result (not in your required format) via aggregation
您可以通过聚合获得结果（不是您所需的格式）
db.collection.aggregate(
{$group : { _id : '$user', count : {$sum : 1}}}
).result
the output for your sample documents is:
示例文档的输出是：
"0" : {
"_id" : "2",
"count" : 1
},
"1" : {
"_id" : "3",
"count" : 1
},
"2" : {
"_id" : "1",
"count" : 2
}
回答by Reece Daniels
For anyone reading this in Jan 2019 the accepted answer does not currently work in Robo3T (returns a pipeline.length - 1error).
对于在 2019 年 1 月阅读本文的任何人，接受的答案目前在 Robo3T 中不起作用（返回pipeline.length - 1错误）。
You must:
你必须：
a) wrap the query in a set of square brackets []
a) 将查询包裹在一组方括号中 []
b) remove .resultfrom the end
b).result从末尾删除
https://github.com/Studio3T/robomongo/issues/1519#issuecomment-441348191
https://github.com/Studio3T/robomongo/issues/1519#issuecomment-441348191
Here's an update to the accepted answer by @disposer that works for me in Robo3T.
这是@disposer 接受的答案的更新，它适用于我在 Robo3T 中。
db.getCollection('collectionName').aggregate(
[ {$group : { _id : '$user', count : {$sum : 1}}} ]
)
回答by chridam
With MongoDb 3.6 and newer, you can leverage the use of $arrayToObjectoperator and a $replaceRootpipeline to get the desired result. You would need to run the following aggregate pipeline:
使用 MongoDb 3.6 和更新版本，您可以利用$arrayToObject运算符和$replaceRoot管道的使用来获得所需的结果。您需要运行以下聚合管道：
db.collection.aggregate([
{ "$group": {
"_id": "$user",
"count": { "$sum": 1 }
} },
{ "$sort": { "_id": 1 } },
{ "$group": {
"_id": null,
"counts": {
"$push": {
"k": "$_id",
"v": "$count"
}
}
} },
{ "$replaceRoot": {
"newRoot": { "$arrayToObject": "$counts" }
} }
])
which yields
这产生
{
"1" : 2,
"2" : 1,
"3" : 1
}
