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
];

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

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

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

Hint

Use countDocuments with genreFilter and assign to fantasyBooks.

Practice

1.

What does the countDocuments method do in MongoDB?

easy

A. Counts how many documents match a given filter

B. Deletes documents matching a filter

C. Updates documents matching a filter

D. Returns all documents in a collection

Solution

1. Step 1: Understand the purpose of countDocuments

   The countDocuments method is used to count documents that match a filter in a collection.

2. Step 2: Compare with other operations

   Deleting, updating, or returning documents are different operations and not related to counting.

3. Final Answer:

   Counts how many documents match a given filter -> Option A

4. Quick Check:

   countDocuments = count matching documents [OK]

Hint: Remember: countDocuments counts, not modifies data [OK]

Common Mistakes:

• Confusing countDocuments with delete or update methods
• Thinking it returns the documents instead of a count
• Assuming it counts all documents without a filter

2.

Which of the following is the correct syntax to count documents with status equal to "active" in a collection named users?

?

easy

A. db.users.countDocuments({ status: "active" })

B. db.users.count({ status: active })

C. db.users.find({ status: "active" }).countDocuments({ status: "active" })

D. db.users.countDocuments("status = active")

Solution

1. Step 1: Identify correct method usage

   The countDocuments method is called on the collection with a filter object inside parentheses.

2. Step 2: Check filter format

   The filter must be an object like { status: "active" }, not a string or chained after find().

3. Final Answer:

   db.users.countDocuments({ status: "active" }) -> Option A

4. Quick Check:

   Correct syntax uses countDocuments(filter) [OK]

Hint: Use countDocuments with a filter object inside parentheses [OK]

Common Mistakes:

• Using deprecated count() method
• Passing filter as a string instead of object
• Calling countDocuments after find()

3.

Given the collection orders with documents:

[{ "status": "shipped" }, { "status": "pending" }, { "status": "shipped" }]

What will db.orders.countDocuments({ status: "shipped" }) return?

medium

A. 1

B. 0

C. 2

D. 3

Solution

1. Step 1: Identify documents matching the filter

   Documents with status: "shipped" are the first and third documents.

2. Step 2: Count matching documents

   There are 2 such documents in total.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Count of shipped orders = 2 [OK]

Hint: Count documents matching filter exactly [OK]

Common Mistakes:

• Counting all documents instead of filtered ones
• Misreading the status values
• Assuming countDocuments returns documents, not count

4.

What is wrong with this code snippet?

const count = db.products.countDocuments("category: 'books'");

It aims to count documents where category is "books".

medium

A. Missing await keyword for asynchronous call

B. The filter is passed as a string instead of an object

C. countDocuments cannot be used on the products collection

D. The method name should be count, not countDocuments

Solution

1. Step 1: Check filter argument type

   The filter must be an object like { category: 'books' }, not a string.

2. Step 2: Confirm method usage

   countDocuments is valid on collections and accepts an object filter.

3. Final Answer:

   The filter is passed as a string instead of an object -> Option B

4. Quick Check:

   Filter must be an object, not a string [OK]

Hint: Filters are objects, not strings in countDocuments [OK]

Common Mistakes:

• Passing filter as a string
• Confusing countDocuments with count
• Ignoring async/await in some environments (not always error)

5.

You want to count how many users have either age greater than 30 or status equal to "active". Which query correctly uses countDocuments to do this?

hard

A. db.users.countDocuments({ age > 30 || status == "active" })

B. db.users.countDocuments({ age: { $gt: 30 }, status: "active" })

C. db.users.countDocuments({ $and: [ { age: { $gt: 30 } }, { status: "active" } ] })

D. db.users.countDocuments({ $or: [ { age: { $gt: 30 } }, { status: "active" } ] })

Solution

1. Step 1: Understand the filter logic

   We want documents where age is greater than 30 OR status is "active".

2. Step 2: Use correct MongoDB query syntax

   The $or operator takes an array of conditions to match either one.

3. Step 3: Check each option

   The option using { $or: [ { age: { $gt: 30 } }, { status: "active" } ] } is correct. The option with comma-separated conditions uses implicit AND, the one with $and uses explicit AND, and the last uses invalid syntax.

4. Final Answer:

   db.users.countDocuments({ $or: [ { age: { $gt: 30 } }, { status: "active" } ] }) -> Option D

5. Quick Check:

   Use $or with array for OR conditions [OK]

Hint: Use $or with array for OR conditions in filters [OK]

Common Mistakes:

• Using implicit AND instead of OR
• Writing invalid filter syntax
• Confusing $and and $or operators