Pretty printing and cursor behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of find() with pretty()

What is the output of the following MongoDB query on a collection with documents {"name": "Alice", "age": 30} and {"name": "Bob", "age": 25}?

MongoDB

db.users.find().pretty()

A SyntaxError: pretty() is not a function

B { "name": "Alice", "age": 30 }, { "name": "Bob", "age": 25 }

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

D

{
  "name": "Alice",
  "age": 30
}
{
  "name": "Bob",
  "age": 25
}

💡 Hint

pretty() formats the output with indentation and line breaks.

The pretty() method formats the cursor output to be more readable by adding line breaks and indentation, showing each document on its own lines but not as a JSON array.

🧠 Conceptual

intermediate

Cursor behavior after iteration

After running the following code, what happens if you try to iterate over the cursor again?

MongoDB

var cursor = db.users.find();
while(cursor.hasNext()) {
  printjson(cursor.next());
}
// Now iterate again
while(cursor.hasNext()) {
  printjson(cursor.next());
}

A The second loop resets the cursor automatically and prints all documents.

B The second loop prints no documents because the cursor is exhausted.

C The second loop throws a runtime error because the cursor is closed.

D The second loop prints the same documents again.

💡 Hint

Think about how cursors work in MongoDB after reading all documents.

Once a cursor is fully iterated, it is exhausted and has no more documents to return. You cannot iterate over it again without creating a new cursor.

📝 Syntax

advanced

Correct usage of pretty() with find()

Which of the following MongoDB shell commands correctly uses pretty() to format the output of a find query?

A db.collection.find().pretty

B db.collection.pretty().find()

C db.collection.find().pretty()

D db.collection.find(pretty())

💡 Hint

pretty() is a method called on the cursor returned by find().

pretty() is a method chained after find() to format the output. Calling pretty() before find() or passing it as an argument is invalid syntax.

❓ optimization

advanced

Improving performance when pretty printing large query results

You want to pretty print a large number of documents from a collection. Which approach is best to avoid performance issues?

A Use find().limit(10).pretty() to print only a small number of documents.

B Use find().pretty() with no limit and print all documents at once.

C Use find().pretty() and store all results in an array before printing.

D Use find() without pretty() and format the output manually in the application.

💡 Hint

Consider how much data is processed and displayed at once.

Limiting the number of documents reduces memory and processing load, making pretty printing faster and more responsive for large datasets.

🔧 Debug

expert

Diagnosing cursor error after pretty() usage

You run this code in the MongoDB shell: var cursor = db.users.find().pretty(); printjson(cursor.next()); printjson(cursor.next()); But you get an error: "TypeError: cursor.next is not a function". What is the cause?

A pretty() returns a string, not a cursor, so next() is undefined.

B The cursor is exhausted after the first next() call, so the second fails.

C The find() method must be called with a query object to use next().

D The pretty() method must be called after next(), not before.

💡 Hint

Check what pretty() returns compared to find().

pretty() formats the output as a string and does not return a cursor. Calling next() on the string causes a TypeError.

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