Find values group by another field in MongoDB?

To group by another field, use $group along with $project. Let us first create a collection with documents −

> db.demo374.insertOne( ...    { ... ...       "Name" : "Chris", ...       "HobbyDetails" : [ ...          "Reading Book", ...          "Playing Football" ...       ], ...       "CountryName" : "US" ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5e5a04402ae06a1609a00b04") } > db.demo374.insertOne( ...    { ... ...       "Name" : "Chris", ...       "HobbyDetails" : [ ...          "Browsing Internet", ...          "Playing Football" ...       ], ...       "CountryName" : "US" ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5e5a04402ae06a1609a00b05") }

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

> db.demo374.find();

This will produce the following output −

{ "_id" : ObjectId("5e5a04402ae06a1609a00b04"), "Name" : "Chris", "HobbyDetails" : [ "Reading Book", "Playing Football" ], "CountryName" : "US" } { "_id" : ObjectId("5e5a04402ae06a1609a00b05"), "Name" : "Chris", "HobbyDetails" : [ "Browsing Internet", "Playing Football" ], "CountryName" : "US" }

Following is the query to find values group by another field in MongoDB −

> db.demo374.aggregate([ ...    {$match : { "CountryName" : "US"}}, ...    {$group : {"_id" : "$Name", "HobbyDetails" : {$addToSet : "$HobbyDetails"}}}, ...    {$project : {"_id" : 0, "Name" : "$_id", "HobbyDetails" : {$reduce : {input : "$HobbyDetails", initialValue : [], in: { $concatArrays : ["$$value", "$$this"] }}}}} ... ]).pretty()

This will produce the following output −

{    "Name" : "Chris",    "HobbyDetails" : [       "Browsing Internet",       "Playing Football",       "Reading Book",       "Playing Football"    ] }