Getting a list of values by using MongoDB $group?

To get a list of values, use $group aggregation along with $push operator. Let us first create a collection with documents −

> db.groupByDemo.insertOne({"UserName":"John","Subject":"MongoDB"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd69f0457806ebf1256f136") } > db.groupByDemo.insertOne({"UserName":"Larry","Subject":"MySQL"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd69f0657806ebf1256f137") } > db.groupByDemo.insertOne({"UserName":"John","Subject":"Java"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd69f0d57806ebf1256f138") } > db.groupByDemo.insertOne({"UserName":"John","Subject":"C"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd69f1357806ebf1256f139") } > db.groupByDemo.insertOne({"UserName":"Larry","Subject":"SQL Server"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd69f1c57806ebf1256f13a") }

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

> db.groupByDemo.find().pretty();

This will produce the following output −

{    "_id" : ObjectId("5cd69f0457806ebf1256f136"),    "UserName" : "John",    "Subject" : "MongoDB" } {    "_id" : ObjectId("5cd69f0657806ebf1256f137"),    "UserName" : "Larry",    "Subject" : "MySQL" } {    "_id" : ObjectId("5cd69f0d57806ebf1256f138"),    "UserName" : "John",    "Subject" : "Java" } {    "_id" : ObjectId("5cd69f1357806ebf1256f139"),    "UserName" : "John",    "Subject" : "C" } {    "_id" : ObjectId("5cd69f1c57806ebf1256f13a"),    "UserName" : "Larry",    "Subject" : "SQL Server" }

Following is the query to get list of values by using $group and $push −

> db.groupByDemo.aggregate({$group: { '_id': '$UserName', 'Subject': { $push: '$Subject'}}});

This will produce the following output −

{ "_id" : "Larry", "Subject" : [ "MySQL", "SQL Server" ] } { "_id" : "John", "Subject" : [ "MongoDB", "Java", "C" ] }