Subset pattern for large documents in MongoDB - Mini Project: Build & Apply

Subset Pattern for Large Documents in MongoDB

📖 Scenario: You work at a library that stores detailed book information in a MongoDB collection. Each book document contains many fields, including a large reviews array with hundreds of entries. To improve performance when showing book summaries, you want to fetch only a subset of fields, excluding the large reviews array.

🎯 Goal: Build a MongoDB query that retrieves only the title, author, and year fields from the books collection, excluding the reviews field to reduce data size.

📋 What You'll Learn

Create a books collection with three book documents including title, author, year, and reviews fields

Define a projection object to exclude the reviews field

Write a MongoDB query using find() with the projection to get only the subset of fields

Complete the query to return the cursor for further processing

💡 Why This Matters

🌍 Real World

Large documents with many fields or arrays can slow down queries and increase network load. Using the subset pattern by excluding large fields helps improve performance when only summary data is needed.

💼 Career

Database developers and administrators often optimize queries by projecting only necessary fields, especially when working with large documents in MongoDB or similar NoSQL databases.

1

Create the books collection with sample documents

Create a variable called books and assign it an array with these three objects exactly: { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald', year: 1925, reviews: ['Excellent', 'Classic', 'Must read'] }, { title: '1984', author: 'George Orwell', year: 1949, reviews: ['Thought-provoking', 'Dystopian', 'Timeless'] }, { title: 'To Kill a Mockingbird', author: 'Harper Lee', year: 1960, reviews: ['Powerful', 'Emotional', 'Important'] }

MongoDB

# Create the books array with three book documents
# Your code here

Need a hint?

Use a list of dictionaries with exact keys and values as shown.

2

Define a projection to exclude the reviews field

Create a variable called projection and set it to an object that excludes the reviews field by setting reviews: 0

MongoDB

books = [
    { 'title': 'The Great Gatsby', 'author': 'F. Scott Fitzgerald', 'year': 1925, 'reviews': ['Excellent', 'Classic', 'Must read'] },
    { 'title': '1984', 'author': 'George Orwell', 'year': 1949, 'reviews': ['Thought-provoking', 'Dystopian', 'Timeless'] },
    { 'title': 'To Kill a Mockingbird', 'author': 'Harper Lee', 'year': 1960, 'reviews': ['Powerful', 'Emotional', 'Important'] }
]
# Define projection to exclude reviews
# Your code here

Need a hint?

Use { 'reviews': 0 } to exclude the reviews field in MongoDB projection.

3

Write the MongoDB find() query with the projection

Create a variable called query and assign it the result of calling db.books.find({}, projection) to fetch all documents but only the subset of fields

MongoDB

books = [
    { 'title': 'The Great Gatsby', 'author': 'F. Scott Fitzgerald', 'year': 1925, 'reviews': ['Excellent', 'Classic', 'Must read'] },
    { 'title': '1984', 'author': 'George Orwell', 'year': 1949, 'reviews': ['Thought-provoking', 'Dystopian', 'Timeless'] },
    { 'title': 'To Kill a Mockingbird', 'author': 'Harper Lee', 'year': 1960, 'reviews': ['Powerful', 'Emotional', 'Important'] }
]
projection = { 'reviews': 0 }
# Write the MongoDB find query with projection
# Your code here

Need a hint?

Use db.books.find({}, projection) to get all documents with the projection applied.

4

Complete the query to return the cursor for further use

Add a line to assign result to the cursor returned by query so it can be used later for iteration

MongoDB

books = [
    { 'title': 'The Great Gatsby', 'author': 'F. Scott Fitzgerald', 'year': 1925, 'reviews': ['Excellent', 'Classic', 'Must read'] },
    { 'title': '1984', 'author': 'George Orwell', 'year': 1949, 'reviews': ['Thought-provoking', 'Dystopian', 'Timeless'] },
    { 'title': 'To Kill a Mockingbird', 'author': 'Harper Lee', 'year': 1960, 'reviews': ['Powerful', 'Emotional', 'Important'] }
]
projection = { 'reviews': 0 }
query = db.books.find({}, projection)
# Assign the result cursor
# Your code here

Need a hint?

Assign the cursor returned by find() to a variable called result.