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.