$unwind for flattening arrays in MongoDB - Mini Project: Build & Apply

$unwind for flattening arrays in MongoDB

📖 Scenario: You work at an online bookstore. Each book document has a title and an array of authors. You want to list each author separately along with the book title.

🎯 Goal: Build a MongoDB aggregation pipeline that uses $unwind to flatten the authors array so each author appears in its own document with the book title.

📋 What You'll Learn

Create a collection called books with documents containing title and authors array

Add a variable for the aggregation pipeline

Use $unwind on the authors field in the pipeline

Complete the aggregation query to return each author with the book title

💡 Why This Matters

🌍 Real World

Flattening arrays in documents is common when you want to analyze or display each item separately, such as listing each author of a book individually.

💼 Career

Database developers and data analysts often use $unwind in MongoDB to prepare data for reporting, filtering, or transforming nested array data.

1

Create the books collection with sample data

Create a variable called books and assign it an array with these exact documents: { title: 'Learn MongoDB', authors: ['Alice', 'Bob'] } and { title: 'Advanced MongoDB', authors: ['Carol'] }.

MongoDB

const books = [
  // Your code here
];

Need a hint?

Use an array of objects with title and authors keys exactly as shown.

2

Create the aggregation pipeline variable

Create a variable called pipeline and assign it an empty array [] to start building the aggregation pipeline.

MongoDB

const books = [
  { title: 'Learn MongoDB', authors: ['Alice', 'Bob'] },
  { title: 'Advanced MongoDB', authors: ['Carol'] }
];

const pipeline = [
  // Your code here
];

Need a hint?

Start with an empty array for the pipeline.

3

Add the $unwind stage to the pipeline

Add an object with { $unwind: '$authors' } to the pipeline array to flatten the authors array.

MongoDB

const books = [
  { title: 'Learn MongoDB', authors: ['Alice', 'Bob'] },
  { title: 'Advanced MongoDB', authors: ['Carol'] }
];

const pipeline = [
  // Your code here
];

Need a hint?

Use { $unwind: '$authors' } inside the pipeline array.

4

Complete the aggregation query using the pipeline

Create a variable called result and assign it the aggregation of books using pipeline with db.collection.aggregate(pipeline) syntax. Assume db.collection is the collection holding books.

MongoDB

const books = [
  { title: 'Learn MongoDB', authors: ['Alice', 'Bob'] },
  { title: 'Advanced MongoDB', authors: ['Carol'] }
];

const pipeline = [
  { $unwind: '$authors' }
];

// Your code here

Need a hint?

Use const result = db.books.aggregate(pipeline); to run the aggregation.