MongoDB聚合和投影
为此，请使用$project和aggregate()。聚合中的$project将带有请求字段的文档传递到管道的下一个阶段。
让我们创建一个包含文档的集合-
> db.demo762.insertOne({
... "_id" : {
... "userId":101,
... "userName":"Chris"
... },
.. . "countryName" : "US",
...
... "details" : [
... {
... "Name" : "Robert",
... "DueDate" : "2020-04-10"
...
... },
...
... {
... "Name" : "Robert",
... "DueDate" : "2020-04-09"
... },
... {
... "Name" : "Robert",
... "DueDate" : "2020-03-06"
... }
... ]
... }
... );
{
"acknowledged" : true,
"insertedId" : {
"userId" : 101,
"userName" : "Chris"
}
}在find()方法的帮助下显示集合中的所有文档-
> db.demo762.find();
这将产生以下输出-
{ "_id" : { "userId" : 101, "userName" : "Chris" }, "countryName" : "US", "details" : [ { "Name" : "Robert", "DueDate" : "2020-04-10" }, { "Name" : "Robert", "DueDate" : "2020-04-09" }, { "Name" : "Robert", "DueDate" : "2020-03-06" } ] }以下是对MongoDB聚合和投影的查询-
> db.demo762.aggregate([
... { "$match": {
... "_id": { "$eq": { userId:101,userName:"Chris" }}
... }},
... { "$unwind": "$details" },
... { "$sort": { "details.DueDate": 1 }},
... { "$group": {
... "_id": "$_id",
... "details": { "$push": "$details" },
... "countryName": { "$first": "$countryName" }
... }},
... { "$project": { "details": { "$slice": ["$details", 2] } ,"countryName": 1 }}
... ]).pretty();这将产生以下输出-
{
"_id" : {
"userId" : 101,
"userName" : "Chris"
},
"countryName" : "US",
"details" : [
{
"Name" : "Robert",
"DueDate" : "2020-03-06"
},
{
"Name" : "Robert",
"DueDate" : "2020-04-09"
}
]
}