Select documents grouped by field in MongoDB?

To select documents grouped by field in MongoDB, use $group along with $project. Let us first create a collection with documents −

> db.demo540.insertOne({id:1,"Name":"Chris","CountryName":"US"});{    "acknowledged" : true, "insertedId" : ObjectId("5e8ca368ef4dcbee04fbbc0e") } > db.demo540.insertOne({id:1,"Name":"Chris","CountryName":"UK"});{    "acknowledged" : true, "insertedId" : ObjectId("5e8ca36bef4dcbee04fbbc0f") } > db.demo540.insertOne({id:1,"Name":"Chris","CountryName":"AUS"});{    "acknowledged" : true, "insertedId" : ObjectId("5e8ca370ef4dcbee04fbbc10") }

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

> db.demo540.find();

This will produce the following output −

{ "_id" : ObjectId("5e8ca368ef4dcbee04fbbc0e"), "id" : 1, "Name" : "Chris", "CountryName" : "US" } { "_id" : ObjectId("5e8ca36bef4dcbee04fbbc0f"), "id" : 1, "Name" : "Chris", "CountryName" : "UK" } { "_id" : ObjectId("5e8ca370ef4dcbee04fbbc10"), "id" : 1, "Name" : "Chris", "CountryName" : "AUS" }

Following is the query to select documents grouped by field −

> var query= [ ... { ...    "$group": { ...       "_id": { ...          "id": "$id", ...          "Name": "$Name" ...       }, ...       "details": { ...          "$push": { ... ...             "CountryName": "$CountryName" ... ...          } ...       } ...    } ... }, ... { ...    "$project": { ...       "_id": "$_id.id", ...          "Name": "$_id.Name", ...             "details": 1 ...       } ...    } ... ]; > db.demo540.aggregate(q).pretty();

This will produce the following output −

{    "details" : [       {          "Name" : "Chris",          "CountryName" : "US"       },       {          "Name" : "Chris",          "CountryName" : "UK"       },       {          "Name" : "Chris",          "CountryName" : "AUS"       }    ],    "_id" : 1,    "Name" : "Chris" }