Mastering Mongoose: A Complete Guide to Common MongoDB ODM Interview Questions

Mongoose is one of the most popular Object Document Mappers (ODM) for MongoDB. As an integral part of many full-stack JavaScript applications built with Node.js and MongoDB, candidates are frequently tested on their Mongoose skills during technical interviews.

In this comprehensive guide, we will explore some of the most common Mongoose MongoDB ODM interview questions that candidates can expect to face, along with detailed explanations and example code snippets to help you master Mongoose.

Before we dive into the interview questions, let’s briefly go over what Mongoose is and why it’s useful:

• Mongoose provides a schema-based solution for modeling MongoDB data in your Node.js application. This includes features like validation, query building, and business logic hooks.

• It allows you to define schemas for documents to enforce structure, validations, defaults, getters, setters and more.

• Mongoose helps translate between objects in code and their representation in MongoDB. This allows for seamless integration and modeling of data.

• It also provides helper functions like querying, updating, deleting data and more in an intuitive manner

• Overall, Mongoose makes it easier to work with MongoDB in a Node.js app by providing a schema-based ODM layer.

Now that we know what Mongoose is, let’s look at some common interview questions on it:

Basic Mongoose Interview Questions

Q1. How do you connect to MongoDB using Mongoose?

To connect to a MongoDB database using Mongoose you use the mongoose.connect() method

const mongoose = require('mongoose');mongoose.connect('mongodb://localhost:27017/mydb', {  useNewUrlParser: true,   useUnifiedTopology: true});

The connection URI contains the database host, port and name. You can also pass additional options like useNewUrlParser and useUnifiedTopology for avoiding deprecation warnings.

Q2. How do you define a Schema in Mongoose?

A Schema represents the structure of a document in MongoDB using Mongoose. Here is how you define a schema:

const mongoose = require('mongoose');const userSchema = new mongoose.Schema({  name: String,  age: Number});

The schema defines the fields or properties of the document along with their data types.

Q3. What is a Mongoose Model and how do you create one?

A Mongoose Model provides an interface for interacting with the database collection. It is created using a defined schema:

const User = mongoose.model('User', userSchema);

The first argument is the singular name of the collection your model is for. Mongoose automatically looks for the plural version of this name (‘Users’ in this case) when storing documents in MongoDB.

Q4. How do you perform CRUD operations using Mongoose?

Mongoose provides helpful methods for creating, retrieving, updating and deleting documents from a collection.

• Create – Model.create()
• Read – Model.find(), Model.findById()
• Update – Model.updateOne(), Model.updateMany(), Model.findByIdAndUpdate()
• Delete – Model.deleteOne(), Model.deleteMany(), Model.findByIdAndDelete()

For example:

// Insert a documentconst user = await User.create({  name: 'John',  age: 20 });// Find by IDconst user = await User.findById(id); // Update by IDawait User.findByIdAndUpdate(id, { age: 22 });// Delete by ID await User.findByIdAndDelete(id);

Q5. How do you create relationships between documents in Mongoose?

There are two main ways to create relationships between documents with Mongoose:

1. Referencing – Store the _id field of one document in another document as a reference. Useful for one-to-many relationships.

2. Embedding – Embed one document entirely within another as a subdocument. Useful for one-to-one and one-to-few relationships.

For example:

// Author embeds many Booksconst authorSchema = new Schema({  name: String,  books: [bookSchema] });// Book references Author const bookSchema = new Schema({  title: String,  author: {    type: Schema.Types.ObjectId,    ref: 'Author'  }});

Q6. Explain Population in Mongoose

Population in Mongoose is the process of automatically replacing the document _id reference in a field with the actual full document from another collection.

For example:

// Get books and populate author detailsBook.find().populate('author').exec((err, books) => {  // books.author now contains the full author document}); 

This avoids the need for making separate queries manually to populate references.

Intermediate Mongoose Interview Questions

Q7. How do you use validation in Mongoose?

Mongoose allows you to define validations at the schema level to ensure data integrity before insertion and updates:

const userSchema = new Schema({  name: {    type: String,    required: true  },  age: {    type: Number,    min: 18,    max: 65  }});

The validators like required, min and max automatically validate the data before a document is saved to the database.

Q8. How are Mongoose middlewares used?

Mongoose provides middleware functions like pre-save and post-save that can be used to hook into the model lifecycle events.

For example:

userSchema.pre('save', function(next) {  // Modifications  next(); });userSchema.post('save', function(doc, next) {  // Notifications  next();});

These are useful for tasks like hashing passwords, formatting data, logging and more that need to happen before or after save operations.

Q9. How do you create indexes in Mongoose?

Indexes can be created on specific fields in a Mongoose schema using the index property:

const userSchema = new Schema({  name: { type: String, index: true },  email: { type: String, unique: true }});

This will create single field indexes on name and a unique compound index on email. Indexes improve query performance.

Q10. How does virtual work in Mongoose?

Virtuals in Mongoose are fields that are computed on-the-fly and not stored in MongoDB. For example:

// Computed fullNameuserSchema.virtual('fullName').get(function() {  return this.firstName + ' ' + this.lastName;});

These can be handy for combining fields, performing transformations, adding methods and more.

Q11. How do you use aggregation pipeline in Mongoose?

Mongoose provides a .aggregate() method on models that allows you to build aggregation pipelines to process data and output aggregated results:

const total = await User.aggregate([    { $group: { _id: null, count: { $sum: 1 } } }]);

This provides full access to MongoDB’s powerful aggregation framework using Mongoose models.

Advanced Mongoose Interview Questions

Q12. How do you handle asynchronous validation in Mongoose?

For asynchronous validators that rely on async operations, Mongoose provides an async validator:

userSchema.path('email').validate(async function(value) {  const emailExists = await checkEmailExists(value);  return !emailExists;}, 'Email already exists');

The validator function can return a Promise to handle async behavior.

Q13. How to select fields returned from a Mongoose query?

The .select() method allows you to choose which fields to return in the documents from a Mongoose query:

User.find().select('name email').exec((err, users) => {  // users only contains name and email});

This is useful for optimizing queries by only returning needed fields from large documents.

Q14. How do you handle transactions in Mongoose?

Mongoose supports MongoDB transactions that allows executing multiple operations in isolation. Steps:

1. Start a transaction session
2. Use session for all operations to be atomic
3. Call session.commitTransaction() to commit, or session.abortTransaction() to rollback

For example:

const session = await mongoose.startSession();session.startTransaction();await Model1.updateOne({...}, { session });await Model2.updateOne({...}, { session });await session