Why I am getting the error "no database found"?

I am using mongodb as my database. I am trying to save the details(title, price and description) of a product to my database.

Here is my database.js code

const mongodb= require('mongodb');

const MongoClient = mongodb.MongoClient;

let _db;

const mongoConnect= callback=>{
    MongoClient.connect('mongodb+srv://pawan:*************@cluster0.vchvo.mongodb.net/myFirstDatabase?retryWrites=true&w=majority')
    .then(client=>{
        console.log('connected');
        _db= client.db();
        callback();
    })
    .catch(err=>{
        console.log(err);
        throw err;
    })
}

const getDb=()=>{
    if(_db){
        return _db;
    }
    throw 'no database found';
};

exports.mongoConnect= mongoConnect;
exports.getDb= getDb;

(I have placed "*******" in place of password only for question). I could say that my database connection has been made as I can see the Connected message on my console.

Here is the save() function implementation in the product.js in models

save() {
        const db = getDb();
        return db.collection('products')
            .insertOne(this)
            .then(result=>{
                console.log(result);
            })
            .catch(err=>{
                console.log("error from save in model"+ err);
            })
    }

Here is the controller implementation:

exports.postAddProduct = (req, res, next) => {
    const title=req.body.title;
    const price=req.body.price;
    const description=req.body.description;

    const product = new Product(title, price, description);
    product.save()
        .then(result => {
            console.log("Created product:  "+ result);
            res.redirect('/');
            }
        )
        .catch(err => {
            console.log(err);
        })

}

Upon running my code when I submit the details of the product instead of being redirected to "/", I am getting a message as "no database found".

Please guide me to resolve this problem.