Set unique index in MongoDB

To set unique index in MongoDB, use unique:true. Let us create a collection with documents −

> db.demo257.ensureIndex({Name:1},{unique:true});
{
   "createdCollectionAutomatically" : true,
   "numIndexesBefore" : 1,
   "numIndexesAfter" : 2,
   "ok" : 1
}
> db.demo257.insertOne({Name:"Chris"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e47a5e51627c0c63e7dba91")
}
> db.demo257.insertOne({Name:"Bob"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e47a5e81627c0c63e7dba92")
}
> db.demo257.insertOne({Name:"Chris"});
2020-02-15T13:33:54.064+0530 E QUERY    [js] WriteError: E11000 duplicate key error collection: test.demo257 index: Name_1 dup key: { : "Chris" } :
WriteError({
   "index" : 0,
   "code" : 11000,
   "errmsg" : "E11000 duplicate key error collection: test.demo257 index: Name_1 dup key: { : \"Chris\" }",
   "op" : {
      "_id" : ObjectId("5e47a5ea1627c0c63e7dba93"),
      "Name" : "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.demo257.find();

This will produce the following output −

{ "_id" : ObjectId("5e47a5e51627c0c63e7dba91"), "Name" : "Chris" }
{ "_id" : ObjectId("5e47a5e81627c0c63e7dba92"), "Name" : "Bob" }