Is it possible to rename _id field after MongoDB group aggregation?

Yes, it is possible to rename using aggregation. Let us first create a collection with documents

> db.renameIdDemo.insertOne({"StudentName":"Chris"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9a1760353decbc2fc927c5") } > db.renameIdDemo.insertOne({"StudentName":"Robert"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9a1765353decbc2fc927c6") } > db.renameIdDemo.insertOne({"StudentName":"David"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9a176b353decbc2fc927c7") }

Following is the query to display all documents from a collection with the help of find() method

> db.renameIdDemo.find();

This will produce the following output

{ "_id" : ObjectId("5c9a1760353decbc2fc927c5"), "StudentName" : "Chris" } { "_id" : ObjectId("5c9a1765353decbc2fc927c6"), "StudentName" : "Robert" } { "_id" : ObjectId("5c9a176b353decbc2fc927c7"), "StudentName" : "David" }

Following is the query to rename _id field:

> db.renameIdDemo.aggregate({ $project: { ...    _id: 0, ...    mainId: "$_id", ...    count: 1, ...    sum: 1 ... } ... } ... );

This will produce the following output. We have renamed _id to mainId;

{ "mainId" : ObjectId("5c9a1760353decbc2fc927c5") } { "mainId" : ObjectId("5c9a1765353decbc2fc927c6") } { "mainId" : ObjectId("5c9a176b353decbc2fc927c7") }