在系统变量$$ROOT上应用组累加器运算符 first，以返回对根文档的引用？
$ROOT引用当前在聚合管道阶段正在处理的根文档，即顶级文档。
让我们创建一个包含文档的集合-
> db.demo582.insertOne({FirstName:"Chris",Age:21,createDate:new ISODate("2020-01-10")});{
"acknowledged" : true, "insertedId" : ObjectId("5e91ce41fd2d90c177b5bcbd")
}
> db.demo582.insertOne({FirstName:"Chris",Age:21,createDate:new ISODate("2020-04-21")});{
"acknowledged" : true, "insertedId" : ObjectId("5e91ce4ffd2d90c177b5bcbe")
}
> db.demo582.insertOne({FirstName:"Chris",Age:22,createDate:new ISODate("2020-02-11")});{
"acknowledged" : true, "insertedId" : ObjectId("5e91ce59fd2d90c177b5bcbf")
}
> db.demo582.insertOne({FirstName:"Chris",Age:22,createDate:new ISODate("2020-01-12")});{
"acknowledged" : true, "insertedId" : ObjectId("5e91ce6efd2d90c177b5bcc0")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo582.find();
这将产生以下输出-
{ "_id" : ObjectId("5e91ce41fd2d90c177b5bcbd"), "FirstName" : "Chris", "Age" : 21, "createDate" : ISODate("2020-01-10T00:00:00Z") }
{ "_id" : ObjectId("5e91ce4ffd2d90c177b5bcbe"), "FirstName" : "Chris", "Age" : 21, "createDate" : ISODate("2020-04-21T00:00:00Z") }
{ "_id" : ObjectId("5e91ce59fd2d90c177b5bcbf"), "FirstName" : "Chris", "Age" : 22, "createDate" : ISODate("2020-02-11T00:00:00Z") }
{ "_id" : ObjectId("5e91ce6efd2d90c177b5bcc0"), "FirstName" : "Chris", "Age" : 22, "createDate" : ISODate("2020-01-12T00:00:00Z") }以下是应用GroupAccumulator运算符的查询-
> db.demo582.aggregate([
... {
... "$group": {
... "_id": "$FirstName",
... "MaximumDate": {
... "$max": "$createDate"
... },
... "count": {
... "$sum": 1
... },
... "details": {
... "$first": "$$ROOT"
... }
... }
... },
... {
... "$project": {
... "MaximumDate": 1,
... "count": 1,
... "details": {
... "_id": "$_id",
... "FirstName": "$details.FirstName",
... "Age" : "$details.Age",
... }
... }
... }
... ])这将产生以下输出-
{ "_id" : "Chris", "MaximumDate" : ISODate("2020-04-21T00:00:00Z"), "count" : 4, "details" :
{ "_id" : "Chris", "FirstName" : "Chris", "Age" : 21 }
}