In MongoDB, the primary key is reserved for the _id field. This is a system field and gets created by default when inserting new records. When you want to enforce the uniqueness in other fields, you can use unique index.

Here is how to create unique index with Mongo Shell. Unique key can be a compound key. You can use getIndexes() to get indexes on the collection.

Now, let’s check out how it can be done with Node.js. In Node, you can use createIndex() as in the Mongo shell. As usual, it is asynchronous and you need a callback function with the usual error-first signature.

When you first run the code, it inserts the records and create unique index on the studentId field. When you run it for the second time and try to insert the same records, you will get an error on insertion because it is violating the unique index rule.

// Setting up
const MongoClient = require('mongodb').MongoClient
const url = 'mongodb://yourUsername:yourPassword@localhost:27017/admin'

// Data
const initialData = [{
    studentId:1,
    firstName:'John',
    lastName:'West',
    Major:'Psychology'
},{
    studentId:2,
    firstName:'John',
    lastName:'Doe',
    Major:'Science'
},{
    studentId:3,
    firstName:'Shane',
    lastName:'West',
    Major:'IT'
}]

// function to create uniqe index
const uniqueIndex = (collection, callback) => {
    collection.createIndex({studentId:1}, {unique:true}, (err, result) => {
        if(err) {console.error(`Failed to create index ${err}`); process.exit(1);}
        console.log(`Unique Index created successfully: ${result}`)
        callback(result)
    })
}

// function to insert student records
const insertStudents= (collection, callback) => {
    collection.insert(initialData, (err, result) => {
        if (err) {
            console.error(`Error in insertion: ${err}`)
            process.exit(1)
        }
        console.log(`No of records (result.result.n): ${result.result.n}`)
        console.log(`No of records (result.ops.length): ${result.ops.length}`)
        callback(result)
    })
}

// put them all together
MongoClient.connect(url, (err, client) => {
    if (err) return process.exit(1)
    console.log('Connection successful.')
    const db = client.db('usermanaged')
    const collection = db.collection('students')
    insertStudents(collection, () => {
        uniqueIndex(collection, () => {
            client.close()
            console.log('Connection closed')
        })
    })
})

REFERENCE

• Unique Indexes on MongoDB