How to add an extra field in a sub document in MongoDB?

You can use update(). Let us first create a collection with documents −

> db.addExtraFieldDemo.insertOne(    {       "_id": 1,       "UserName": "Larry" ,       "UserOtherDetails":[          {             "UserCountryName": "US",             "UserAge":23          },          {             "UserCountryName": "UK",             "UserAge":24          }       ]    } ); { "acknowledged" : true, "insertedId" : 1 }

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

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

This will produce the following output −

{    "_id" : 1,    "UserName" : "Larry",    "UserOtherDetails" : [       {          "UserCountryName" : "US",          "UserAge" : 23       },       {          "UserCountryName" : "UK",          "UserAge" : 24       }    ] }

Following is the query to add an extra field in a sub document in MongoDB −

> db.addExtraFieldDemo.update({ "_id":1},{ "$set":{ "UserOtherDetails.0.isMarried_" : true }}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Let us check the documents once again −

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

This will produce the following output −

{    "_id" : 1,    "UserName" : "Larry",    "UserOtherDetails" : [       {          "UserCountryName" : "US",          "UserAge" : 23,          "isMarried_" : true       },       {          "UserCountryName" : "UK",          "UserAge" : 24       }    ] }