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" }