$lookup for joining collections in MongoDB - Mini Project: Build & Apply

$lookup for joining collections

📖 Scenario: You work at a small online bookstore. You have two collections: books and authors. Each book has an author_id that matches the _id of an author. You want to combine information from both collections to see each book with its author's details.

🎯 Goal: Build a MongoDB aggregation query using $lookup to join the books collection with the authors collection, so each book document includes its author's information.

📋 What You'll Learn

Create a books collection with three books, each having _id, title, and author_id fields.

Create an authors collection with three authors, each having _id and name fields.

Write an aggregation query on books using $lookup to join with authors on author_id and _id.

Add a $project stage to show only title and the joined author name.

💡 Why This Matters

🌍 Real World

Joining collections is common when you have related data in different places, like customers and orders or products and suppliers.

💼 Career

Understanding $lookup is essential for MongoDB developers to combine data efficiently without duplicating it.

1

Create the books collection

Create a books collection with these exact documents: { _id: 1, title: "The Great Gatsby", author_id: 101 }, { _id: 2, title: "1984", author_id: 102 }, and { _id: 3, title: "To Kill a Mockingbird", author_id: 103 }.

MongoDB

// Insert the three book documents into the books collection
// Your code here

Need a hint?

Use db.books.insertMany() with an array of objects for the three books.

2

Create the authors collection

Create an authors collection with these exact documents: { _id: 101, name: "F. Scott Fitzgerald" }, { _id: 102, name: "George Orwell" }, and { _id: 103, name: "Harper Lee" }.

MongoDB

db.books.insertMany([
  { _id: 1, title: "The Great Gatsby", author_id: 101 },
  { _id: 2, title: "1984", author_id: 102 },
  { _id: 3, title: "To Kill a Mockingbird", author_id: 103 }
])
// Insert the three author documents into the authors collection
// Your code here

Need a hint?

Use db.authors.insertMany() with an array of author objects.

3

Write the aggregation query with $lookup

Write an aggregation query on books using db.books.aggregate() with a $lookup stage that joins authors on author_id from books matching _id from authors. Use as: "author_info" to store the joined data.

MongoDB

db.books.insertMany([
  { _id: 1, title: "The Great Gatsby", author_id: 101 },
  { _id: 2, title: "1984", author_id: 102 },
  { _id: 3, title: "To Kill a Mockingbird", author_id: 103 }
])
db.authors.insertMany([
  { _id: 101, name: "F. Scott Fitzgerald" },
  { _id: 102, name: "George Orwell" },
  { _id: 103, name: "Harper Lee" }
])

// Write the aggregation query with $lookup below
// Your code here

Need a hint?

Use $lookup inside db.books.aggregate() with the correct fields and collection names.

4

Add $project to show book title and author name

Add a $project stage after $lookup in the aggregation pipeline to show only the title field and the first element's name from author_info as author.

MongoDB

db.books.insertMany([
  { _id: 1, title: "The Great Gatsby", author_id: 101 },
  { _id: 2, title: "1984", author_id: 102 },
  { _id: 3, title: "To Kill a Mockingbird", author_id: 103 }
])
db.authors.insertMany([
  { _id: 101, name: "F. Scott Fitzgerald" },
  { _id: 102, name: "George Orwell" },
  { _id: 103, name: "Harper Lee" }
])
db.books.aggregate([
  {
    $lookup: {
      from: "authors",
      localField: "author_id",
      foreignField: "_id",
      as: "author_info"
    }
  },
  // Add $project stage here
  // Your code here
])

Need a hint?

Use $project to keep title and extract the first author's name from author_info array.