Type conversion expressions ($toInt, $toString) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert the field 'age' to an integer using $toInt.

MongoDB

{ $project: { ageInt: { [1]: "$age" } } }

Drag options to blanks, or click blank then click option'

A$toString

B$toDate

C$toDouble

D$toInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $toString instead of $toInt.

Using $toDouble which converts to double, not integer.

The $toInt operator converts a value to an integer type.

2fill in blank

medium

Complete the code to convert the field 'score' to a string using $toString.

MongoDB

{ $project: { scoreStr: { [1]: "$score" } } }

Drag options to blanks, or click blank then click option'

A$toString

B$toInt

C$toBool

D$toDecimal

Attempts:

3 left

💡 Hint

Common Mistakes

Using $toInt which converts to integer, not string.

Using $toBool which converts to boolean, not string.

The $toString operator converts a value to a string type.

3fill in blank

hard

Fix the error in the code to convert 'price' to integer using $toInt.

MongoDB

{ $project: { priceInt: { [1]: "price" } } }

Drag options to blanks, or click blank then click option'

A$toString

B$toInt

C$toDouble

D$toDate

Attempts:

3 left

💡 Hint

Common Mistakes

Not prefixing the field name with '$'.

Using $toString instead of $toInt.

The field name must be prefixed with '$' to refer to the field value. The operator $toInt is correct for integer conversion.

4fill in blank

hard

Fill both blanks to convert 'quantity' to integer and 'description' to string.

MongoDB

{ $project: { qtyInt: { [1]: "$quantity" }, descStr: { [2]: "$description" } } }

Drag options to blanks, or click blank then click option'

A$toInt

B$toString

C$toDouble

D$toBool

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the operators for the fields.

Using $toDouble or $toBool incorrectly.

$toInt converts 'quantity' to integer, and $toString converts 'description' to string.

5fill in blank

hard

Fill all three blanks to convert 'height' to integer, 'weight' to string, and 'active' to boolean.

MongoDB

{ $project: { heightInt: { [1]: "$height" }, weightStr: { [2]: "$weight" }, isActive: { [3]: "$active" } } }

Drag options to blanks, or click blank then click option'

A$toInt

B$toString

C$toBool

D$toDouble

Attempts:

3 left

💡 Hint

Common Mistakes

Using $toDouble instead of $toBool for boolean conversion.

Mixing up $toInt and $toString.

$toInt converts 'height' to integer, $toString converts 'weight' to string, and $toBool converts 'active' to boolean.

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