Restrict Inserting Duplicate Items in MongoDB

For this, use ensureIndex() and set unique:true. Let us create a collection with documents. Here, when we try to inert duplicate item, then a duplicate key error occurs −

> db.demo316.ensureIndex({"SubjectName":1},{unique:true});
{
   "createdCollectionAutomatically" : true,
   "numIndexesBefore" : 1,
   "numIndexesAfter" : 2,
   "ok" : 1
}
> db.demo316.insertOne({"SubjectName":"MySQL"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e50e378f8647eb59e56205d")
}
> db.demo316.insertOne({"SubjectName":"MongoDB"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e50e37df8647eb59e56205e")
}
> db.demo316.insertOne({"SubjectName":"MongoDB"});
2020-02-22T13:47:05.186+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: test.demo316 index: SubjectName_1 dup key: { : "MongoDB" } :
WriteError({
   "index" : 0,
   "code" : 11000,
   "errmsg" : "E11000 duplicate key error collection: test.demo316 index: SubjectName_1 dup key: { : \"MongoDB\" }",
   "op" : {
      "_id" : ObjectId("5e50e381f8647eb59e56205f"),
      "SubjectName" : "MongoDB"
   }
})
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.demo316.find();

This will produce the following output −

{ "_id" : ObjectId("5e50e378f8647eb59e56205d"), "SubjectName" : "MySQL" }
{ "_id" : ObjectId("5e50e37df8647eb59e56205e"), "SubjectName" : "MongoDB" }