Add field that is unique index to collection in MongoDB?

For unique index, set unique − true while creating an index. Let us create a collection with documents −

> db.demo658.createIndex({FirstName:1},{unique:true,sparse:true});
{
   "createdCollectionAutomatically" : true,
   "numIndexesBefore" : 1,
   "numIndexesAfter" : 2,
   "ok" : 1
}
>
> db.demo658.insertOne({"FirstName":"John","LastName":"Smith"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea067144deddd72997713d9")
}
> db.demo658.insertOne({"FirstName":"Adam","LastName":"Smith"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea0671c4deddd72997713da")
}
> db.demo658.insertOne({"FirstName":"John","LastName":"Doe"});
2020-04-22T21:17:46.072+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: onlinecustomertracker.demo658 index: FirstName_1 dup key: { : "John" } :
WriteError({
   "index" : 0,
   "code" : 11000,
   "errmsg" : "E11000 duplicate key error collection: onlinecustomertracker.demo658 index: FirstName_1 dup key: { : \"John\" }",
   "op" : {
      "_id" : ObjectId("5ea067224deddd72997713db"),
      "FirstName" : "John",
      "LastName" : "Doe"
   }
})
WriteError@src/mongo/shell/bulk_api.js:461:48
Bulk/mergeBatchResults@src/mongo/shell/bulk_api.js:841:49
Bulk/executeBatch@src/mongo/shell/bulk_api.js:906:13
Bulk/this.execute@src/mongo/shell/bulk_api.js:1150:21
DBCollection.prototype.insertOne@src/mongo/shell/crud_api.js:252:9
@(shell):1:1

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

> db.demo658.find();

This will produce the following output −

{ "_id" : ObjectId("5ea067144deddd72997713d9"), "FirstName" : "John", "LastName" : "Smith" }
{ "_id" : ObjectId("5ea0671c4deddd72997713da"), "FirstName" : "Adam", "LastName" : "Smith" }