Type conversion expressions ($toInt, $toString) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this MongoDB aggregation stage?

Consider the following aggregation pipeline stage applied to a collection with documents like { "value": "123" }:

{ "$project": { "intValue": { "$toInt": "$value" } } }

What will be the value of intValue in the output document?

MongoDB

{ "$project": { "intValue": { "$toInt": "$value" } } }

A { "intValue": null }

B { "intValue": "123" }

C { "intValue": 123 }

D Error: Cannot convert string to int

💡 Hint

The $toInt operator converts a string that looks like a number into an integer.

The $toInt operator converts the string "123" into the integer 123. So the output document will have intValue as a number, not a string.

❓ query_result

intermediate

What happens when $toInt converts a floating-point number?

Given a document { "score": 98.7 }, what is the result of this aggregation stage?

{ "$project": { "intScore": { "$toInt": "$score" } } }

MongoDB

{ "$project": { "intScore": { "$toInt": "$score" } } }

A Error: Cannot convert float to int

B { "intScore": 98 }

C { "intScore": 98.7 }

D { "intScore": 99 }

💡 Hint

$toInt truncates the decimal part when converting floats.

The $toInt operator truncates the decimal part of a floating-point number, so 98.7 becomes 98.

📝 Syntax

advanced

Which aggregation expression correctly converts a field to string?

You want to convert the field count to a string in an aggregation pipeline. Which of the following expressions is syntactically correct and will work?

A { "$toString": count }

B { "$toStr": "$count" }

C { "$convert": { "input": "$count", "to": "string" } }

D { "$toString": "$count" }

💡 Hint

The operator name is $toString and the field must be referenced as a string with $.

Option D uses the correct operator $toString and references the field with "$count". Option D uses a non-existent operator. Option D is valid syntax but uses $convert, which is different from $toString. Option D misses quotes around the field reference.

🔧 Debug

advanced

Why does this aggregation stage cause an error?

Given documents with field price as strings like "12.99", why does this pipeline stage fail?

{ "$project": { "priceInt": { "$toInt": "$price" } } }

MongoDB

{ "$project": { "priceInt": { "$toInt": "$price" } } }

A Because $toInt cannot convert strings with decimal points

B Because $toInt requires numeric input, not strings

C Because the field name must be without quotes

D Because $toInt only works on integers, not floats

💡 Hint

Check how $toInt handles strings with decimal points.

$toInt cannot convert strings that represent decimal numbers like "12.99" directly, causing an error.

🧠 Conceptual

expert

Which statement about $toInt and $toString is true?

Select the correct statement about MongoDB's $toInt and $toString operators in aggregation pipelines.

A $toInt throws an error if the input cannot be converted exactly to an integer

B $toString only works on numeric types, not on arrays or objects

C $toString converts any BSON type to its string representation without error

D $toInt can convert strings representing integers and floats by truncating decimals

💡 Hint

Think about error handling and input types for these operators.

Option A is true: $toInt throws an error if the input cannot be converted exactly to an integer (e.g., strings with decimals). Option A is false because $toInt cannot convert floats represented as strings directly. Option A is false because $toString may fail on some BSON types. Option A is false because $toString can convert many BSON types including arrays and objects.

Practice

1. What does the $toInt expression do in MongoDB aggregation?

easy

A. Converts a value to a date

B. Converts a value to an integer (whole number)

C. Converts a value to a boolean (true/false)

D. Converts a value to a string (text)

Solution

1. Step 1: Understand the purpose of $toInt

   The $toInt operator converts values to integers, which are whole numbers without decimals.

2. Step 2: Compare with other conversion types

   $toString converts to text, $toBool to boolean, and $toDate to date. So $toInt specifically converts to integer.

3. Final Answer:

   Converts a value to an integer (whole number) -> Option B

4. Quick Check:

   $toInt = integer conversion [OK]

Hint: Remember: Int means whole number, String means text [OK]

Common Mistakes:

• Confusing $toInt with $toString
• Thinking $toInt converts to decimal numbers
• Mixing up $toInt with boolean conversion

2. Which of the following is the correct syntax to convert a field age to string in a MongoDB aggregation pipeline?

easy

A. { $toInt: "$age" }

B. { $toString: age }

C. { $toString: "$age" }

D. { toString: "$age" }

Solution

1. Step 1: Identify correct operator and field reference

   The operator for string conversion is $toString. Field names must be referenced as strings with a $ prefix inside quotes, like "$age".

2. Step 2: Check syntax correctness

   { $toString: "$age" } uses correct operator and field reference. { $toString: age } misses quotes around field. { $toInt: "$age" } uses wrong operator $toInt. { toString: "$age" } misses $ sign before operator.

3. Final Answer:

   { $toString: "$age" } -> Option C

4. Quick Check:

   Use $ and quotes for fields, $toString for string conversion [OK]

Hint: Use $ before field names and operators in MongoDB [OK]

Common Mistakes:

• Omitting quotes around field names
• Forgetting $ before operator
• Using wrong operator for conversion

3. Given the document { "score": "85" }, what will be the result of this aggregation expression: { $toInt: "$score" }?

medium

A. 85 (as an integer)

B. "85" (as a string)

C. null

D. Error: Cannot convert string to int

Solution

1. Step 1: Understand input and conversion

   The field "score" is a string "85". Using $toInt converts this string to the integer 85.

2. Step 2: Confirm output type

   The output will be the number 85, not a string or error, because "85" is a valid numeric string.

3. Final Answer:

   85 (as an integer) -> Option A

4. Quick Check:

   String "85" to int = 85 [OK]

Hint: Numeric strings convert cleanly to integers [OK]

Common Mistakes:

• Expecting output to remain a string
• Thinking conversion causes error
• Confusing null with error

4. You wrote this aggregation expression: { $toInt: 123abc }. What is the problem?

medium

A. It returns null silently

B. It converts to integer 123

C. It converts to string "123abc"

D. Syntax error because 123abc is not a valid number or string

Solution

1. Step 1: Analyze the input value

   The value 123abc is not a valid number or quoted string. MongoDB expects a number or string literal for conversion.

2. Step 2: Understand MongoDB parsing rules

   Without quotes, 123abc is invalid syntax, causing a syntax error.

3. Final Answer:

   Syntax error because 123abc is not a valid number or string -> Option D

4. Quick Check:

   Invalid literals cause syntax errors [OK]

Hint: Always quote strings; invalid literals cause errors [OK]

Common Mistakes:

• Assuming partial conversion works
• Forgetting to quote strings
• Expecting silent null return

5. You have a collection with documents like { "value": 123 } and { "value": "456" }. You want to create a new field valueStr that is always a string. Which aggregation expression correctly converts value to string regardless of original type?

hard

A. { $toString: "$value" }

B. { $toInt: "$value" }

C. { $toString: value }

D. { $toString: { $toInt: "$value" } }

Solution

1. Step 1: Understand the data types

   Some values are numbers (123), some are strings ("456"). We want to convert all to strings.

2. Step 2: Choose correct conversion

   $toString converts any type to string. Using { $toString: "$value" } converts both numbers and strings to string type.

3. Step 3: Evaluate other options

   { $toInt: "$value" } converts to int, not string. { $toString: value } misses quotes around field name. { $toString: { $toInt: "$value" } } converts to int then string, unnecessary and may cause errors if string is non-numeric.

4. Final Answer:

   { $toString: "$value" } -> Option A

5. Quick Check:

   Use $toString with quoted field for consistent string conversion [OK]

Hint: Use $toString with "$field" to convert any type to string [OK]

Common Mistakes:

• Missing quotes around field name
• Using $toInt instead of $toString
• Unnecessary double conversion causing errors