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Apply the Group Accumulator Operator $first on the system variable $$ROOT to return reference to the root document?
The accumulators are operators that maintain their state as documents progress through the pipeline.
The $ROOT references the root document, i.e. the top-level document, currently being processed in the aggregation pipeline stage.
Let us create a collection with documents −
> db.demo582.insertOne({FirstName:"Chris",Age:21,createDate:new ISODate("2020-01-10")});{
"acknowledged" : true, "insertedId" : ObjectId("5e91ce41fd2d90c177b5bcbd")
}
> db.demo582.insertOne({FirstName:"Chris",Age:21,createDate:new ISODate("2020-04-21")});{
"acknowledged" : true, "insertedId" : ObjectId("5e91ce4ffd2d90c177b5bcbe")
}
> db.demo582.insertOne({FirstName:"Chris",Age:22,createDate:new ISODate("2020-02-11")});{
"acknowledged" : true, "insertedId" : ObjectId("5e91ce59fd2d90c177b5bcbf")
}
> db.demo582.insertOne({FirstName:"Chris",Age:22,createDate:new ISODate("2020-01-12")});{
"acknowledged" : true, "insertedId" : ObjectId("5e91ce6efd2d90c177b5bcc0")
}Display all documents from a collection with the help of find() method −
> db.demo582.find();
This will produce the following output −
{ "_id" : ObjectId("5e91ce41fd2d90c177b5bcbd"), "FirstName" : "Chris", "Age" : 21, "createDate" : ISODate("2020-01-10T00:00:00Z") }
{ "_id" : ObjectId("5e91ce4ffd2d90c177b5bcbe"), "FirstName" : "Chris", "Age" : 21, "createDate" : ISODate("2020-04-21T00:00:00Z") }
{ "_id" : ObjectId("5e91ce59fd2d90c177b5bcbf"), "FirstName" : "Chris", "Age" : 22, "createDate" : ISODate("2020-02-11T00:00:00Z") }
{ "_id" : ObjectId("5e91ce6efd2d90c177b5bcc0"), "FirstName" : "Chris", "Age" : 22, "createDate" : ISODate("2020-01-12T00:00:00Z") }Following is the query to apply the Group Accumulator operator −
> db.demo582.aggregate([
... {
... "$group": {
... "_id": "$FirstName",
... "MaximumDate": {
... "$max": "$createDate"
... },
... "count": {
... "$sum": 1
... },
... "details": {
... "$first": "$$ROOT"
... }
... }
... },
... {
... "$project": {
... "MaximumDate": 1,
... "count": 1,
... "details": {
... "_id": "$_id",
... "FirstName": "$details.FirstName",
... "Age" : "$details.Age",
... }
... }
... }
... ])This will produce the following output −
{ "_id" : "Chris", "MaximumDate" : ISODate("2020-04-21T00:00:00Z"), "count" : 4, "details" :
{ "_id" : "Chris", "FirstName" : "Chris", "Age" : 21 }
}
Updated on: 15-May-2020
103 Views
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