Projection for reducing data transfer in MongoDB - Mini Project: Build & Apply

Projection for Reducing Data Transfer in MongoDB

📖 Scenario: You work for a small online bookstore. The database has a collection called books with many details about each book, including title, author, year, price, and description. You want to show a list of books on the website but only need the title and author to reduce the amount of data sent from the database.

🎯 Goal: Build a MongoDB query that uses projection to return only the title and author fields from the books collection, excluding all other fields.

📋 What You'll Learn

Create a books collection with 3 documents containing title, author, year, price, and description fields.

Create a variable called projection that includes only title and author fields.

Write a query using find() on books with the projection to get only those fields.

Add a final command to convert the query result to an array.

💡 Why This Matters

🌍 Real World

Projection helps reduce the amount of data sent from the database to the application, making web pages load faster and saving bandwidth.

💼 Career

Database developers and backend engineers use projection to optimize queries and improve application performance.

1

Create the books collection with sample data

Create a variable called books and assign it an array with these 3 documents exactly: { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, price: 15.99, description: "A fantasy novel." }, { title: "1984", author: "George Orwell", year: 1949, price: 12.99, description: "A dystopian novel." }, { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, price: 14.99, description: "A novel about racial injustice." }

MongoDB

# Create the books array with 3 documents
# Your code here

Hint

Use an array of objects with the exact fields and values given.

2

Create the projection variable to select only title and author

Create a variable called projection and set it to an object that includes title and author with value 1 to include them in the query result.

MongoDB

books = [
    { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, price: 15.99, description: "A fantasy novel." },
    { title: "1984", author: "George Orwell", year: 1949, price: 12.99, description: "A dystopian novel." },
    { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, price: 14.99, description: "A novel about racial injustice." }
]

# Create projection object to include only title and author
# Your code here

Hint

Use 1 to include fields in MongoDB projection.

3

Write the query using find() with the projection

Write a query using db.books.find({}, projection) to get only the title and author fields from the books collection.

MongoDB

books = [
    { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, price: 15.99, description: "A fantasy novel." },
    { title: "1984", author: "George Orwell", year: 1949, price: 12.99, description: "A dystopian novel." },
    { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, price: 14.99, description: "A novel about racial injustice." }
]

projection = { title: 1, author: 1 }

# Write the query to find all books with projection
# Your code here

Hint

Use empty filter {} to select all documents and pass projection as second argument.

4

Convert the query result to an array

Add .toArray() to the query to get the results as an array and assign it to a variable called result.

MongoDB

books = [
    { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, price: 15.99, description: "A fantasy novel." },
    { title: "1984", author: "George Orwell", year: 1949, price: 12.99, description: "A dystopian novel." },
    { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, price: 14.99, description: "A novel about racial injustice." }
]

projection = { title: 1, author: 1 }

query = db.books.find({}, projection)

# Convert query to array and assign to result
# Your code here

Hint

Use .toArray() to get all documents from the cursor as an array.

Practice

1. What is the main purpose of using projection in a MongoDB query?

easy

A. To select only specific fields from documents to reduce data transfer

B. To sort the documents in ascending order

C. To delete documents from the collection

D. To update multiple documents at once

Solution

1. Step 1: Understand projection concept

   Projection in MongoDB is used to specify which fields to include or exclude in the query result.

2. Step 2: Identify the purpose of projection

   Projection reduces the amount of data sent by selecting only needed fields, improving speed and efficiency.

3. Final Answer:

   To select only specific fields from documents to reduce data transfer -> Option A

4. Quick Check:

   Projection = Select fields only [OK]

Hint: Projection picks fields to reduce data size [OK]

Common Mistakes:

• Confusing projection with sorting
• Thinking projection deletes documents
• Assuming projection updates data

2. Which of the following is the correct syntax to include only the name and age fields in a MongoDB find query?

easy

A. db.collection.find({}, {name: 1, age: 1})

B. db.collection.find({name: 1, age: 1})

C. db.collection.find({}, {name: 0, age: 0})

D. db.collection.find({name: 0, age: 0})

Solution

1. Step 1: Recall find() method syntax

   The first argument is the filter (empty {} means all documents), the second is the projection specifying fields to include or exclude.

2. Step 2: Identify correct projection for inclusion

   Using 1 includes fields; so {name: 1, age: 1} includes only those fields.

3. Final Answer:

   db.collection.find({}, {name: 1, age: 1}) -> Option A

4. Quick Check:

   Include fields = 1 in second argument [OK]

Hint: Use 1 to include fields in projection [OK]

Common Mistakes:

• Putting projection in the first argument
• Using 0 to include fields
• Omitting the empty filter {}

3. Given the collection documents:

{"_id": 1, "name": "Alice", "age": 25, "city": "NY"}

{"_id": 2, "name": "Bob", "age": 30, "city": "LA"}

What will be the output of this query?
db.collection.find({}, {name: 1, _id: 0})

medium

A. [{"_id": 1, "name": "Alice"}, {"_id": 2, "name": "Bob"}]

B. [{"age": 25, "city": "NY"}, {"age": 30, "city": "LA"}]

C. [{"name": "Alice"}, {"name": "Bob"}]

D. [{"name": "Alice", "age": 25}, {"name": "Bob", "age": 30}]

Solution

1. Step 1: Analyze projection fields

   The projection includes only name with 1 and excludes _id with 0, so only the name field appears without _id.

2. Step 2: Determine output documents

   Each document will have only the name field, no _id or other fields.

3. Final Answer:

   [{"name": "Alice"}, {"name": "Bob"}] -> Option C

4. Quick Check:

   Projection includes name only, excludes _id [OK]

Hint: Set _id:0 to exclude _id field [OK]

Common Mistakes:

• Forgetting _id is included by default
• Expecting other fields to appear
• Using 0 to include fields

4. Identify the error in this MongoDB query that tries to exclude the password field:
db.users.find({}, {password: 1, email: 0})

medium

A. Projection must include _id explicitly

B. The filter argument is missing

C. The query is correct and will work as expected

D. You cannot mix including and excluding fields in projection except for _id

Solution

1. Step 1: Understand projection rules

   MongoDB does not allow mixing inclusion (1) and exclusion (0) in the same projection except for the _id field.

2. Step 2: Analyze the given projection

   The projection tries to include password (1) and exclude email (0) together, which is invalid.

3. Final Answer:

   You cannot mix including and excluding fields in projection except for _id -> Option D

4. Quick Check:

   Mixing 1 and 0 in projection = error [OK]

Hint: Include or exclude fields, not both (except _id) [OK]

Common Mistakes:

• Mixing 1 and 0 in projection
• Assuming _id must be included explicitly
• Ignoring filter argument is optional

5. You want to retrieve only the title and author fields from a large books collection but exclude the _id field to reduce data size. Which query correctly achieves this and why?

hard

A. db.books.find({}, {title: 0, author: 0, _id: 1}) because excluding fields reduces data

B. db.books.find({}, {title: 1, author: 1, _id: 0}) because it includes needed fields and excludes _id

C. db.books.find({}, {title: 1, author: 1}) because _id is excluded by default

D. db.books.find({}, {title: 1, author: 0, _id: 0}) because mixing inclusion and exclusion is allowed

Solution

1. Step 1: Determine fields to include and exclude

   You want to include title and author fields and exclude _id to reduce data size.

2. Step 2: Apply correct projection syntax

   Including fields uses 1, excluding _id uses 0. Mixing inclusion and exclusion (except _id) is not allowed.

3. Step 3: Evaluate options

   db.books.find({}, {title: 1, author: 1, _id: 0}) because it includes needed fields and excludes _id correctly includes title and author with 1 and excludes _id with 0. Others either exclude needed fields or mix inclusion/exclusion incorrectly.

4. Final Answer:

   db.books.find({}, {title: 1, author: 1, _id: 0}) because it includes needed fields and excludes _id -> Option B

5. Quick Check:

   Include fields with 1, exclude _id with 0 [OK]

Hint: Include needed fields with 1, exclude _id with 0 [OK]

Common Mistakes:

• Assuming _id is excluded by default
• Mixing inclusion and exclusion fields
• Excluding needed fields by mistake