Type conversion expressions ($toInt, $toString) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $toInt expression do in MongoDB?

It converts a value to an integer type. For example, a string like "123" becomes the number 123.

beginner

How does $toString work in MongoDB?

It converts a value to a string type. For example, the number 456 becomes the string "456".

intermediate

Why would you use $toInt in a MongoDB aggregation pipeline?

To make sure values are treated as numbers for calculations or comparisons, especially if they are stored as strings.

intermediate

What happens if $toInt tries to convert a non-numeric string?

It results in an error or null depending on the context, because the string cannot be converted to a number.

beginner

Give an example of using $toString in a MongoDB aggregation.

Example: { $toString: "$age" } converts the numeric field age to a string.

What type does $toInt convert a value into?

A String

B Array

C Boolean

D Integer

$toInt converts values to integers.

Which expression converts a number to a string in MongoDB?

A $toString

B $toInt

C $toBool

D $toArray

$toString converts values to strings.

What will { $toInt: "123" } return?

A 123

B null

C Error

D "123"

The string "123" converts to the integer 123.

If $toInt tries to convert "abc", what happens?

A Returns 0

B Returns "abc"

C Error or null

D Returns true

Non-numeric strings cause an error or null when converted to int.

Why use $toString in aggregation?

A To perform math operations

B To convert numbers to strings for display or concatenation

C To filter documents

D To sort documents

$toString helps convert numbers to strings for display or joining text.

Explain how and why you would use $toInt and $toString in a MongoDB aggregation pipeline.

Think about data types and what operations need numbers or strings.

What are possible issues when converting types with $toInt and how can you handle them?

Consider what happens if the data is not in the expected format.

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