Type conversion expressions ($toInt, $toString) in MongoDB - Step-by-Step Execution

Concept Flow - Type conversion expressions ($toInt, $toString)

Start with input value

↓

Apply $toInt or $toString

↓

Check if conversion is possible

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Return converted

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Output

The flow shows how MongoDB converts a value to integer or string using $toInt or $toString, checking if conversion is possible and returning the converted value or error.

Execution Sample

MongoDB

[
  { $project: { numberAsInt: { $toInt: "$numberStr" }, numberAsStr: { $toString: "$number" } } }
]

This aggregation stage converts a string field 'numberStr' to integer and an integer field 'number' to string.

Execution Table

Step	Input Value	Expression	Conversion Result	Notes
1	"123"	$toInt	123	String '123' converts to integer 123
2	123	$toString	"123"	Integer 123 converts to string '123'
3	"abc"	$toInt	Error or null	String 'abc' cannot convert to integer
4	true	$toString	"true"	Boolean true converts to string 'true'
5	null	$toInt	null	Null converts to null for $toInt
6	45.67	$toInt	45	Float 45.67 converts to integer 45 by truncation
7	"45.67"	$toInt	Error or null	String '45.67' cannot convert directly to int
8	Exit	-	-	No more conversions

💡 Execution stops after all input values are processed or conversion fails.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	Final
inputValue	"123"	123	"abc"	true	null	45.67	"45.67"	Exit	Exit
convertedValue	N/A	123	"123"	Error/null	"true"	null	45	Error/null	End

Key Moments - 3 Insights

Why does converting the string '45.67' with $toInt fail while the float 45.67 converts successfully?

Because $toInt can convert numeric types like float by truncation, but cannot convert strings with decimal points directly, causing an error or null (see execution_table row 6 and 7).

What happens when $toInt tries to convert a non-numeric string like 'abc'?

It results in an error or null because the string does not represent a valid number (see execution_table row 3).

How does $toString handle boolean values?

$toString converts boolean true or false to their string equivalents 'true' or 'false' (see execution_table row 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the conversion result of input value "123" with $toInt?

A 123

B "123"

C Error or null

D null

💡 Hint

Check execution_table row 1 under Conversion Result.

At which step does $toInt conversion fail due to invalid string input?

A Step 2

B Step 3

C Step 6

D Step 4

💡 Hint

Look for 'Error or null' in Conversion Result column in execution_table.

If the input value is boolean true, what does $toString convert it to?

A "true"

B true

C 1

D Error

💡 Hint

See execution_table row 4 Conversion Result.

Concept Snapshot

$toInt and $toString convert values to integer or string.
Use $toInt to convert strings or numbers to integers (floats truncate).
Use $toString to convert any value to string.
Invalid conversions return error or null.
Useful in aggregation pipelines for type consistency.

Full Transcript

This visual execution trace shows how MongoDB's $toInt and $toString expressions convert values. Starting with an input value, the expression attempts conversion. If successful, the converted value is returned; otherwise, an error or null is returned. Examples include converting string '123' to integer 123, integer 123 to string '123', and handling invalid conversions like string 'abc' to integer which fails. The trace highlights how floats convert by truncation and booleans convert to their string forms. This helps beginners understand step-by-step how type conversions behave in MongoDB aggregation pipelines.