$size operator for array length in MongoDB - Mini Project: Build & Apply

$size Operator for Array Length in MongoDB

📖 Scenario: You are managing a database for a book club. Each book document stores the title and a list of members who have read it.

🎯 Goal: Build a MongoDB query that finds books with exactly 3 readers using the $size operator.

📋 What You'll Learn

Create a collection named books with documents containing title and readers array fields.

Insert 3 book documents with specific titles and readers arrays.

Write a query using the $size operator to find books with exactly 3 readers.

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

💡 Why This Matters

🌍 Real World

Managing book club data where you want to find books read by a specific number of members.

💼 Career

Filtering documents by array length is common in MongoDB for tasks like user activity tracking, inventory management, and social media data analysis.

1

Create the books collection with 3 documents

Insert 3 documents into the books collection with these exact entries: { title: "The Hobbit", readers: ["Alice", "Bob", "Charlie"] }, { title: "1984", readers: ["Dave", "Eve"] }, and { title: "Dune", readers: ["Frank", "Grace", "Heidi"] }.

MongoDB

// Insert the 3 book documents into the books collection
// Your code here

Need a hint?

Use db.books.insertMany([...]) to add multiple documents at once.

2

Set the target array length to 3

Create a variable called targetLength and set it to 3 to represent the number of readers to filter by.

MongoDB

db.books.insertMany([
  { title: "The Hobbit", readers: ["Alice", "Bob", "Charlie"] },
  { title: "1984", readers: ["Dave", "Eve"] },
  { title: "Dune", readers: ["Frank", "Grace", "Heidi"] }
])

// Create a variable targetLength and set it to 3
// Your code here

Need a hint?

Use const targetLength = 3 to store the number 3.

3

Write a query using $size to find books with 3 readers

Write a MongoDB query using db.books.find() with a filter that uses { readers: { $size: targetLength } } to find books where the readers array length is exactly 3.

MongoDB

db.books.insertMany([
  { title: "The Hobbit", readers: ["Alice", "Bob", "Charlie"] },
  { title: "1984", readers: ["Dave", "Eve"] },
  { title: "Dune", readers: ["Frank", "Grace", "Heidi"] }
])

const targetLength = 3;

// Write a query to find books with readers array length equal to targetLength
// Your code here

Need a hint?

Use { readers: { $size: targetLength } } inside find() to filter by array length.

4

Add a projection to show only the title field

Modify the query to include a projection that returns only the title field by adding { title: 1, _id: 0 } as the second argument to db.books.find().

MongoDB

db.books.insertMany([
  { title: "The Hobbit", readers: ["Alice", "Bob", "Charlie"] },
  { title: "1984", readers: ["Dave", "Eve"] },
  { title: "Dune", readers: ["Frank", "Grace", "Heidi"] }
])

const targetLength = 3;

db.books.find({ readers: { $size: targetLength } }, { title: 1, _id: 0 })
// Your code here

Need a hint?

Add { title: 1, _id: 0 } as the second argument to find() to show only titles.