Multikey indexes for arrays in MongoDB - Mini Project: Build & Apply

Create and Query Multikey Indexes for Arrays in MongoDB

📖 Scenario: You are managing a MongoDB collection that stores information about books in a library. Each book document contains a title and an array of authors. You want to create an index on the authors array to speed up searches for books by author.

🎯 Goal: Build a MongoDB collection with book documents containing an array of authors. Create a multikey index on the authors field. Then write a query to find all books by a specific author.

📋 What You'll Learn

Create a collection called books with documents having title and authors fields

Insert three book documents with exact titles and authors arrays

Create a multikey index on the authors field

Write a query to find all books where authors array contains 'Jane Austen'

💡 Why This Matters

🌍 Real World

Libraries, bookstores, and content platforms often store multiple authors per book. Multikey indexes help quickly find books by any author.

💼 Career

Database developers and administrators use multikey indexes to optimize queries on array fields in MongoDB, improving application performance.

1

Create the books collection with three book documents

Insert three documents into the books collection with these exact fields and values: { title: 'Pride and Prejudice', authors: ['Jane Austen'] }, { title: 'Good Omens', authors: ['Neil Gaiman', 'Terry Pratchett'] }, and { title: 'The Hobbit', authors: ['J.R.R. Tolkien'] }.

MongoDB

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

Need a hint?

Use db.books.insertMany() with an array of three objects, each having title and authors fields.

2

Create a multikey index on the authors field

Create an index on the authors field of the books collection using db.books.createIndex().

MongoDB

db.books.insertMany([
  { title: 'Pride and Prejudice', authors: ['Jane Austen'] },
  { title: 'Good Omens', authors: ['Neil Gaiman', 'Terry Pratchett'] },
  { title: 'The Hobbit', authors: ['J.R.R. Tolkien'] }
])
// Create a multikey index on authors
// Your code here

Need a hint?

Use db.books.createIndex({ authors: 1 }) to create the multikey index.

3

Write a query to find books by 'Jane Austen'

Write a query using db.books.find() to find all documents where the authors array contains the string 'Jane Austen'.

MongoDB

db.books.insertMany([
  { title: 'Pride and Prejudice', authors: ['Jane Austen'] },
  { title: 'Good Omens', authors: ['Neil Gaiman', 'Terry Pratchett'] },
  { title: 'The Hobbit', authors: ['J.R.R. Tolkien'] }
])
db.books.createIndex({ authors: 1 })
// Find books where authors array contains 'Jane Austen'
// Your code here

Need a hint?

Use db.books.find({ authors: 'Jane Austen' }) to find matching documents.

4

Add a projection to show only the title field in the query result

Modify the previous db.books.find() query to include a projection that returns only the title field and excludes the _id field.

MongoDB

db.books.insertMany([
  { title: 'Pride and Prejudice', authors: ['Jane Austen'] },
  { title: 'Good Omens', authors: ['Neil Gaiman', 'Terry Pratchett'] },
  { title: 'The Hobbit', authors: ['J.R.R. Tolkien'] }
])
db.books.createIndex({ authors: 1 })
// Modify the find query to show only title and exclude _id
// Your code here

Need a hint?

Add a second argument to db.books.find() with { title: 1, _id: 0 } to project only the title.