countDocuments method in MongoDB - Mini Project: Build & Apply

Counting Documents in a MongoDB Collection

📖 Scenario: You are managing a small library database. You want to find out how many books are in the collection and how many books belong to a specific genre.

🎯 Goal: Build a MongoDB query using the countDocuments method to count all books and then count books of a specific genre.

📋 What You'll Learn

Create a collection named books with sample book documents

Add a filter variable for the genre to count

Use countDocuments to count all books

Use countDocuments with the genre filter to count books of that genre

💡 Why This Matters

🌍 Real World

Counting documents is useful to know how many records match certain criteria in databases like MongoDB, such as counting users, orders, or products.

💼 Career

Database developers and data analysts often need to count records efficiently to generate reports and insights.

1

Create the books collection with sample documents

Create a variable called books that is an array containing these exact documents: { title: "The Hobbit", genre: "Fantasy" }, { title: "1984", genre: "Dystopian" }, { title: "The Catcher in the Rye", genre: "Fiction" }, { title: "The Lord of the Rings", genre: "Fantasy" }.

MongoDB

const books = [
  // Your code here
];

Need a hint?

Use an array of objects with the exact titles and genres given.

2

Add a filter variable for the genre

Create a variable called genreFilter and set it to an object that filters for books with genre equal to "Fantasy".

MongoDB

const books = [
  { title: "The Hobbit", genre: "Fantasy" },
  { title: "1984", genre: "Dystopian" },
  { title: "The Catcher in the Rye", genre: "Fiction" },
  { title: "The Lord of the Rings", genre: "Fantasy" }
];

// Your code here

Need a hint?

Create an object with the key genre and value "Fantasy".

3

Count all books using countDocuments

Write a line that uses db.collection('books').countDocuments() to count all documents and assign the result to a variable called totalBooks.

MongoDB

const books = [
  { title: "The Hobbit", genre: "Fantasy" },
  { title: "1984", genre: "Dystopian" },
  { title: "The Catcher in the Rye", genre: "Fiction" },
  { title: "The Lord of the Rings", genre: "Fantasy" }
];

const genreFilter = { genre: "Fantasy" };

// Your code here

Need a hint?

Use db.collection('books').countDocuments() and assign it to totalBooks.

4

Count books of the genre using countDocuments with filter

Write a line that uses db.collection('books').countDocuments(genreFilter) to count documents matching the genre filter and assign the result to a variable called fantasyBooks.

MongoDB

const books = [
  { title: "The Hobbit", genre: "Fantasy" },
  { title: "1984", genre: "Dystopian" },
  { title: "The Catcher in the Rye", genre: "Fiction" },
  { title: "The Lord of the Rings", genre: "Fantasy" }
];

const genreFilter = { genre: "Fantasy" };

const totalBooks = db.collection('books').countDocuments();

// Your code here

Need a hint?

Use countDocuments with genreFilter and assign to fantasyBooks.