$addToSet accumulator for unique arrays in MongoDB - Mini Project: Build & Apply

$addToSet accumulator for unique arrays

📖 Scenario: You are managing a database for a book club. Each member can borrow multiple books, and you want to keep track of all unique book titles borrowed by each member.

🎯 Goal: Build an aggregation pipeline that groups the data by member and collects a unique list of all book titles they have borrowed using the $addToSet accumulator.

📋 What You'll Learn

Create a collection named borrowedBooks with documents containing member and bookTitle fields.

Add a configuration variable for the aggregation pipeline stages.

Write an aggregation query that groups by member and uses $addToSet to collect unique bookTitle values.

Complete the aggregation pipeline with a final stage to format the output.

💡 Why This Matters

🌍 Real World

Tracking unique items borrowed or purchased by users is common in libraries, stores, and online platforms.

💼 Career

Understanding $addToSet helps in writing efficient aggregation queries for data analysis and reporting in MongoDB.

1

Create the borrowedBooks collection with sample data

Create a collection called borrowedBooks and insert these exact documents: { member: "Alice", bookTitle: "Moby Dick" }, { member: "Alice", bookTitle: "1984" }, { member: "Bob", bookTitle: "Moby Dick" }, { member: "Alice", bookTitle: "Moby Dick" }, { member: "Bob", bookTitle: "The Hobbit" }.

MongoDB

// Insert the specified documents into the borrowedBooks collection
// Your code here

Need a hint?

Use db.collection.insertMany() to add multiple documents at once.

2

Define the aggregation pipeline variable

Create a variable called pipeline and assign it an array with one object that groups documents by member.

MongoDB

db.borrowedBooks.insertMany([
  { member: "Alice", bookTitle: "Moby Dick" },
  { member: "Alice", bookTitle: "1984" },
  { member: "Bob", bookTitle: "Moby Dick" },
  { member: "Alice", bookTitle: "Moby Dick" },
  { member: "Bob", bookTitle: "The Hobbit" }
])

// Create the pipeline variable with a $group stage grouping by member
// Your code here

Need a hint?

Use $group with _id: "$member" to group by member.

3

Add $addToSet to collect unique book titles

Modify the $group stage in the pipeline variable to add a field called uniqueBooks that uses $addToSet to collect unique bookTitle values.

MongoDB

db.borrowedBooks.insertMany([
  { member: "Alice", bookTitle: "Moby Dick" },
  { member: "Alice", bookTitle: "1984" },
  { member: "Bob", bookTitle: "Moby Dick" },
  { member: "Alice", bookTitle: "Moby Dick" },
  { member: "Bob", bookTitle: "The Hobbit" }
])

const pipeline = [
  { $group: {
      _id: "$member",
      // Add uniqueBooks field here using $addToSet
  } }
]

Need a hint?

Inside $group, add uniqueBooks: { $addToSet: "$bookTitle" }.

4

Add a $project stage to format the output

Add a $project stage to the pipeline variable that renames _id to member and includes uniqueBooks in the output.

MongoDB

db.borrowedBooks.insertMany([
  { member: "Alice", bookTitle: "Moby Dick" },
  { member: "Alice", bookTitle: "1984" },
  { member: "Bob", bookTitle: "Moby Dick" },
  { member: "Alice", bookTitle: "Moby Dick" },
  { member: "Bob", bookTitle: "The Hobbit" }
])

const pipeline = [
  { $group: {
      _id: "$member",
      uniqueBooks: { $addToSet: "$bookTitle" }
  } },
  // Add $project stage here to rename _id to member
]

Need a hint?

Use $project to rename _id to member and include uniqueBooks.