String and number types in MongoDB - Mini Project: Build & Apply

Working with String and Number Types in MongoDB

📖 Scenario: You are managing a small bookstore's inventory database using MongoDB. Each book has a title, author, and price. You want to store this information correctly using string and number types.

🎯 Goal: Create a MongoDB collection document for a book with proper string and number types. Then add a discount rate as a number and calculate the discounted price.

📋 What You'll Learn

Create a document with fields title and author as strings

Add a price field as a number

Add a discountRate field as a number representing a percentage

Calculate the discountedPrice by applying the discount rate to the price

💡 Why This Matters

🌍 Real World

Managing product data in a database requires correct use of string and number types to store names, prices, and calculations like discounts.

💼 Career

Database developers and data engineers often work with documents that mix strings and numbers and must perform calculations and updates on stored data.

1

Create the initial book document

Create a MongoDB document called book with these exact fields and values: title set to "The Great Gatsby", author set to "F. Scott Fitzgerald", and price set to 20.99 (a number).

MongoDB

// Create the book document with title, author, and price
// Your code here

Hint

Use curly braces to create an object. Strings go in quotes, numbers do not.

2

Add a discount rate

Add a new field discountRate to the book document and set it to 15 (a number representing 15%).

MongoDB

const book = {
  title: "The Great Gatsby",
  author: "F. Scott Fitzgerald",
  price: 20.99
};
// Add discountRate field to book
// Your code here

Hint

Add the new field inside the object with a comma after the previous field.

3

Calculate the discounted price

Create a new variable called discountedPrice that calculates the price after applying the discount rate. Use the formula: price minus (price times discountRate divided by 100).

MongoDB

const book = {
  title: "The Great Gatsby",
  author: "F. Scott Fitzgerald",
  price: 20.99,
  discountRate: 15
};
// Calculate discountedPrice
// Your code here

Hint

Use arithmetic operators and access the fields with dot notation.

4

Add discounted price to the book document

Add the discountedPrice field to the book document and set its value to the discountedPrice variable you calculated.

MongoDB

const book = {
  title: "The Great Gatsby",
  author: "F. Scott Fitzgerald",
  price: 20.99,
  discountRate: 15
};

const discountedPrice = book.price - (book.price * book.discountRate / 100);
// Add discountedPrice field to book
// Your code here

Hint

Add the new field inside the object with a comma after the previous field.

Practice

1. Which of the following is the correct way to store a string value in a MongoDB document?

easy

A. Use single quotes only, like 'Hello'

B. Write the text without quotes, like Hello

C. Write the text as a number, like 123

D. Use quotes around the text, like "Hello"

Solution

1. Step 1: Understand string representation in MongoDB

   Strings must be enclosed in quotes to be recognized as text.

2. Step 2: Identify correct syntax for strings

   Double quotes or single quotes can be used, but quotes are necessary around text.

3. Final Answer:

   Use quotes around the text, like "Hello" -> Option D

4. Quick Check:

   Strings need quotes = C [OK]

Hint: Strings always need quotes around text in MongoDB [OK]

Common Mistakes:

• Writing text without quotes causes errors
• Confusing numbers with strings
• Using numbers when text is needed

2. Which of the following is the correct way to store the number 42 in a MongoDB document?

easy

A. Write it as '42' with single quotes

B. Write it as "42" with quotes

C. Write it as 42 without quotes

D. Write it as forty-two without quotes

Solution

1. Step 1: Understand number representation in MongoDB

   Numbers are stored without quotes to be recognized as numeric values.

2. Step 2: Identify correct syntax for numbers

   Writing numbers without quotes stores them as numeric types, not strings.

3. Final Answer:

   Write it as 42 without quotes -> Option C

4. Quick Check:

   Numbers have no quotes = A [OK]

Hint: Numbers never have quotes in MongoDB documents [OK]

Common Mistakes:

• Putting numbers inside quotes makes them strings
• Using words instead of digits for numbers
• Mixing string and number types

3. What will be the output of this MongoDB query?

db.products.find({ price: 100 })

Assuming the collection has documents:

{ "name": "Pen", "price": 100 }
{ "name": "Book", "price": "100" }

medium

A. Returns only the document where price is number 100

B. Returns both documents because price matches 100

C. Returns only the document where price is string "100"

D. Returns no documents because of type mismatch

Solution

1. Step 1: Understand MongoDB type matching in queries

   MongoDB matches both value and type when querying, so number 100 matches only number 100.

2. Step 2: Analyze the documents

   Document with price as number 100 matches; document with price as string "100" does not.

3. Final Answer:

   Returns only the document where price is number 100 -> Option A

4. Quick Check:

   Query matches value and type = B [OK]

Hint: MongoDB matches type and value in queries [OK]

Common Mistakes:

• Assuming string and number values match
• Ignoring type differences in queries
• Expecting all similar values to match

4. You wrote this MongoDB insert command:

db.users.insertOne({ "age": "30" })

But you want age stored as a number. What is wrong?

medium

A. The insertOne command syntax is incorrect

B. Age is stored as a string because of quotes around 30

C. The field name should be "Age" with capital A

D. Age is stored as a number but should be a string

Solution

1. Step 1: Check the value type in the insert command

   Quotes around 30 make it a string, not a number.

2. Step 2: Understand how to store numbers

   To store as number, write 30 without quotes.

3. Final Answer:

   Age is stored as a string because of quotes around 30 -> Option B

4. Quick Check:

   Quotes make value string = D [OK]

Hint: Remove quotes to store numbers, add quotes for strings [OK]

Common Mistakes:

• Thinking quotes don't affect type
• Changing field name unnecessarily
• Assuming syntax error without checking value type

5. You have a collection with documents:

{ "item": "apple", "quantity": "10" }
{ "item": "banana", "quantity": 10 }

You want to find all documents where quantity is greater than 5. Which query works correctly?

hard

A. db.collection.find({ quantity: { $gt: NumberInt(5) } })

B. db.collection.find({ quantity: { $gt: "5" } })

C. db.collection.find({ quantity: { $gt: 5 } })

D. db.collection.find({ quantity: { $gt: 5 } }).toArray()

Solution

1. Step 1: Understand type comparison in MongoDB queries

   MongoDB compares values and types; string "10" is not greater than number 5.

2. Step 2: Use NumberInt to ensure numeric comparison

   Using NumberInt(5) ensures the query compares numbers, matching numeric quantity fields.

3. Final Answer:

   db.collection.find({ quantity: { $gt: NumberInt(5) } }) -> Option A

4. Quick Check:

   Use numeric type for numeric comparison = A [OK]

Hint: Use numeric types in queries to compare numbers correctly [OK]

Common Mistakes:

• Comparing strings with numbers directly
• Using quotes around numbers in queries
• Assuming all quantity fields are same type