Prevent duplicates of multiple fields with index in MongoDB

To prevent duplicates of multiple fields, use ensureIndex() and set unique:true. Let us create a collection with documents −

> db.demo272.ensureIndex({"FirstName":1,"Subject":1},{unique:true}); {    "createdCollectionAutomatically" : true,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 } > db.demo272.insertOne({"FirstName":"Chris","Subject":"MySQL"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e48232a1627c0c63e7dbabf") } > db.demo272.insertOne({"FirstName":"Chris","Subject":"MongoDB"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e48232f1627c0c63e7dbac0") } > db.demo272.insertOne({"FirstName":"David","Subject":"MySQL"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e48233a1627c0c63e7dbac1") } > db.demo272.insertOne({"FirstName":"Chris","Subject":"MySQL"}); 2020-02-15T22:28:55.137+0530 E QUERY    [js] WriteError: E11000 duplicate key error collection: test.demo272 index: FirstName_1_Subject_1 dup key: { : "Chris", : "MySQL" } : WriteError({    "index" : 0,    "code" : 11000,    "errmsg" : "E11000 duplicate key error collection: test.demo272 index: FirstName_1_Subject_1 dup key: { : \"Chris\", : \"MySQL\" }",    "op" : {       "_id" : ObjectId("5e48234f1627c0c63e7dbac2"),       "FirstName" : "Chris",       "Subject" : "MySQL"    } }) 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.demo272.find();

This will produce the following output −

{ "_id" : ObjectId("5e48232a1627c0c63e7dbabf"), "FirstName" : "Chris", "Subject" : "MySQL" } { "_id" : ObjectId("5e48232f1627c0c63e7dbac0"), "FirstName" : "Chris", "Subject" : "MongoDB" } { "_id" : ObjectId("5e48233a1627c0c63e7dbac1"), "FirstName" : "D okavid", "Subject" : "MySQL" }