Pretty printing and cursor behavior in MongoDB - Mini Project: Build & Apply

Pretty Printing and Cursor Behavior in MongoDB

📖 Scenario: You are managing a small library database using MongoDB. You want to list all books in a readable format and understand how to work with the cursor that MongoDB returns when you query the collection.

🎯 Goal: Learn how to pretty print query results in MongoDB shell and how to use the cursor to iterate over documents.

📋 What You'll Learn

Create a collection called books with specific documents

Use a variable to hold the cursor returned by a query

Use the pretty() method to display documents in a readable format

Use the cursor's hasNext() and next() methods to iterate documents

💡 Why This Matters

🌍 Real World

Managing and querying collections in MongoDB is common in web applications, content management, and data analysis.

💼 Career

Understanding cursor behavior and pretty printing helps developers and database administrators efficiently retrieve and display data.

1

Create the books collection with sample documents

Insert three documents into the books collection with these exact fields and values: { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937 }, { title: "1984", author: "George Orwell", year: 1949 }, and { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960 }.

MongoDB

// Insert the three specified documents into the books collection
// Your code here

Hint

Use db.books.insertMany() with an array of objects for the three books.

2

Assign the query result to a cursor variable

Create a variable called cursor and assign it the result of db.books.find() to get all documents from the books collection.

MongoDB

db.books.insertMany([
  { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937 },
  { title: "1984", author: "George Orwell", year: 1949 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960 }
])

// Assign the result of db.books.find() to a variable named cursor
// Your code here

Hint

Use var cursor = db.books.find() to store the cursor.

3

Use pretty() to display documents nicely

Call the pretty() method on the cursor variable to display all documents in a readable format.

MongoDB

db.books.insertMany([
  { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937 },
  { title: "1984", author: "George Orwell", year: 1949 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960 }
])

var cursor = db.books.find()

// Use pretty() on cursor to display documents nicely
// Your code here

Hint

Call pretty() on the cursor variable like cursor.pretty().

4

Iterate over the cursor using hasNext() and next()

Write a while loop that uses cursor.hasNext() to check if there are more documents, and inside the loop call cursor.next() to get each document.

MongoDB

db.books.insertMany([
  { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937 },
  { title: "1984", author: "George Orwell", year: 1949 },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960 }
])

var cursor = db.books.find()
cursor.pretty()

// Use a while loop with cursor.hasNext() and cursor.next() to iterate documents
// Your code here

Hint

Use while (cursor.hasNext()) { cursor.next() } to loop through all documents.

Practice

1. What does the pretty() method do when used after a MongoDB query?

easy

A. Formats the output to be more readable with indentation and line breaks

B. Limits the number of documents returned by the query

C. Sorts the documents in ascending order

D. Deletes the documents matching the query

Solution

1. Step 1: Understand the purpose of pretty()

   The pretty() method formats the output of a query to make it easier to read by adding indentation and line breaks.

2. Step 2: Compare with other options

   Limiting, sorting, or deleting documents are done by other methods like limit(), sort(), or remove(), not pretty().

3. Final Answer:

   Formats the output to be more readable with indentation and line breaks -> Option A

4. Quick Check:

   pretty() improves readability [OK]

Hint: Remember: pretty() just makes output look nicer [OK]

Common Mistakes:

• Confusing pretty() with limit() or sort()
• Thinking pretty() changes the data
• Assuming pretty() affects query results count

2. Which of the following is the correct syntax to check if a MongoDB cursor has more documents to iterate?

easy

A. cursor.hasMore()

B. cursor.hasNext()

C. cursor.nextExists()

D. cursor.more()

Solution

1. Step 1: Recall cursor methods in MongoDB

   The correct method to check if a cursor has more documents is hasNext().

2. Step 2: Verify other options

   Methods like hasMore(), nextExists(), or more() do not exist in MongoDB cursor API.

3. Final Answer:

   cursor.hasNext() -> Option B

4. Quick Check:

   Use hasNext() to check cursor availability [OK]

Hint: Use hasNext() to check cursor's next document [OK]

Common Mistakes:

• Using non-existent cursor methods
• Confusing hasNext() with next()
• Assuming hasNext() returns the document itself

3. Given the following MongoDB shell commands, what will be the output?

var cursor = db.users.find({age: {$gt: 25}}).pretty();
while(cursor.hasNext()) {
  printjson(cursor.next());
}

medium

A. No output because cursor.next() is missing parentheses

B. Syntax error because pretty() cannot be chained with find()

C. All user documents with age greater than 25 printed in readable JSON format

D. Only the first user document with age greater than 25 printed

Solution

1. Step 1: Analyze the query and cursor usage

   The query finds users with age > 25 and applies pretty() to format output. The while loop uses hasNext() and next() to print each document.

2. Step 2: Understand the output behavior

   Each matching document is printed in readable JSON format until no documents remain.

3. Final Answer:

   All user documents with age greater than 25 printed in readable JSON format -> Option C

4. Quick Check:

   Cursor iterates all matching docs with pretty print [OK]

Hint: pretty() formats output; hasNext() and next() iterate all [OK]

Common Mistakes:

• Thinking pretty() breaks chaining
• Assuming only one document prints
• Forgetting to call next() as a function

4. You run this code in MongoDB shell:

var cursor = db.products.find().pretty();
if(cursor.hasNext) {
  printjson(cursor.next());
}

What is the problem with this code?

medium

A. hasNext is used without parentheses, so it does not check correctly

B. pretty() cannot be used with find()

C. next() should be called before hasNext()

D. printjson() cannot print cursor documents

Solution

1. Step 1: Identify method usage errors

   The method hasNext is used without parentheses, so it refers to the function itself, not its boolean result.

2. Step 2: Understand impact on code behavior

   Because hasNext is not called, the if condition always evaluates to true (function exists), causing unexpected behavior.

3. Final Answer:

   hasNext is used without parentheses, so it does not check correctly -> Option A

4. Quick Check:

   Always call hasNext() with parentheses [OK]

Hint: Call hasNext() with () to get boolean result [OK]

Common Mistakes:

• Using hasNext without parentheses
• Thinking pretty() causes error
• Calling next() before checking hasNext()

5. You want to print all documents from the orders collection where status is "shipped" in a readable format, but only 3 at a time to avoid memory overload. Which code snippet correctly achieves this?

hard

A. var cursor = db.orders.find({status: "shipped"}).batchSize(3); cursor.pretty(); while(cursor.hasNext()) { printjson(cursor.next()); }

B. var cursor = db.orders.find({status: "shipped"}).pretty(); for(var i=0; i<3; i++) { printjson(cursor.next()); }

C. var cursor = db.orders.find({status: "shipped"}).pretty(); while(cursor.hasNext()) { printjson(cursor.next()); if(i==3) break; }

D. var cursor = db.orders.find({status: "shipped"}).limit(3).pretty(); while(cursor.hasNext()) { printjson(cursor.next()); }

Solution

1. Step 1: Limit query results to 3 documents

   Using limit(3) restricts the cursor to only 3 documents matching the filter.

2. Step 2: Use pretty() to format output and iterate with hasNext()/next()

   Chaining pretty() formats output. The while loop prints each document until the 3 limited documents are exhausted.

3. Final Answer:

   var cursor = db.orders.find({status: "shipped"}).limit(3).pretty(); while(cursor.hasNext()) { printjson(cursor.next()); } -> Option D

4. Quick Check:

   limit(3) + pretty() + hasNext()/next() prints 3 formatted docs [OK]

Hint: Use limit(3) before pretty() and iterate with hasNext()/next() [OK]

Common Mistakes:

• Not using limit() to restrict documents
• Calling pretty() after iteration instead of chaining
• Incorrect loop control causing infinite or missing output