Retrieving group by result with arrays in MongoDB?

To retrieve group by result with arrays, use aggregate(). We will also use $addToSet operator. It adds a value to an array unless the value is already present, in which case $addToSet does nothing to that array.

Let us create a collection with documents −

> db.demo498.insertOne({id:1,Name:["Chris"]});{    "acknowledged" : true,    "insertedId" : ObjectId("5e86192b987b6e0e9d18f553") } > db.demo498.insertOne({id:2,Name:["David"]});{    "acknowledged" : true,    "insertedId" : ObjectId("5e86192d987b6e0e9d18f554") } > db.demo498.insertOne({id:3,Name:["Chris"]});{    "acknowledged" : true,    "insertedId" : ObjectId("5e861931987b6e0e9d18f555") } > db.demo498.insertOne({id:4,Name:["Bob"]});{    "acknowledged" : true,    "insertedId" : ObjectId("5e861942987b6e0e9d18f556") } > db.demo498.insertOne({id:5,Name:["David"]});{    "acknowledged" : true,    "insertedId" : ObjectId("5e861947987b6e0e9d18f557") }

Display all documents from a collection with the help of find() method −

> db.demo498.find();

This will produce the following output −

{ "_id" : ObjectId("5e86192b987b6e0e9d18f553"), "id" : 1, "Name" : [ "Chris" ] } { "_id" : ObjectId("5e86192d987b6e0e9d18f554"), "id" : 2, "Name" : [ "David" ] } { "_id" : ObjectId("5e861931987b6e0e9d18f555"), "id" : 3, "Name" : [ "Chris" ] } { "_id" : ObjectId("5e861942987b6e0e9d18f556"), "id" : 4, "Name" : [ "Bob" ] } { "_id" : ObjectId("5e861947987b6e0e9d18f557"), "id" : 5, "Name" : [ "David" ] }

Following is the query to retrieve group by result with arrays in MongoDB −

> db.demo498.aggregate([{ ...    $unwind : "$Name" ... }, { ...       $group : { ...          _id : $quot;$Name", ...          Id : { ...             $addToSet : "$id" ...          } ...       } ...    } ... ])

This will produce the following output −

{ "_id" : "David", "Id" : [ 5, 2 ] } { "_id" : "Bob", "Id" : [ 4 ] } { "_id" : "Chris", "Id" : [ 3, 1 ] }