How to make a unique field in MongoDB?

To make a unique field in MongoDB, use unique − true. Let us create a collection with documents −

> db.demo82.createIndex({"EmployeeName":1},{unique:true}); {    "createdCollectionAutomatically" : true,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 } > db.demo82.insertOne({"EmployeeName":"Chris"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e2bfb1b71bf0181ecc422a0") } > db.demo82.insertOne({"EmployeeName":"David"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e2bfb1f71bf0181ecc422a1") } > db.demo82.insertOne({"EmployeeName":"Chris"}); 2020-01-25T13:54:00.410+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: test.demo82 index: EmployeeName_1 dup key: { : "Chris" } : WriteError({    "index" : 0,    "code" : 11000,    "errmsg" : "E11000 duplicate key error collection: test.demo82 index: EmployeeName_1 dup key: { : \"Chris\" }",    "op" : {       "_id" : ObjectId("5e2bfb2071bf0181ecc422a2"),       "EmployeeName" : "Chris"    } }) 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.demo82.find();

This will produce the following output. Since, we use unique − true above, therefore the duplicate values won’t get insert −

{ "_id" : ObjectId("5e2bfb1b71bf0181ecc422a0"), "EmployeeName" : "Chris" } { "_id" : ObjectId("5e2bfb1f71bf0181ecc422a1"), "EmployeeName" : "David" }