MongoDB $toLower Operator

The $toLower operator converts string values to lowercase within MongoDB’s aggregation pipeline to lowercase within MongoDB’s aggregation pipeline, enabling consistent text normalization and case-insensitive comparisons for reliable search and formatting.

Converts all alphabetic characters in a string to lowercase.
Accepts any expression that resolves to a string.
Returns an empty string ("") if the input resolves to null.
Behavior depends on MongoDB’s collation and Unicode handling; for language-specific or locale-aware transformations, use collation.
Designed for aggregation stages only (e.g., $project, $addFields, $match with $expr), not for direct update operations.

Syntax

{ $toLower: <expression> }

<expression>: Any valid expression that evaluates to a string.
If the expression resolves to null, $toLower returns an empty string ("").

Examples of Using MongoDB $toLower Operator

In the following examples, we are working with:

Database: GeeksforGeeks
Collection: employee
Document: Three documents that contain the details of the employees in the form of field-value pairs.

Example 1: Using $toLower Operator

Convert the value of the department field in lowercase.

db.employee.aggregate([
  { $match: { "name.first": "Diego" } },
  { $project: { dept: { $toLower: "$department" } } }
])

Output:

Filters the document where name.first is "Diego".
Converts department to lowercase using $toLower.
Outputs the transformed value as dept.

Example 2: Using $toLower on nested fields (via dot notation)

Convert the name.first field to lowercase for employees in the "Development" department

db.employee.aggregate([
  { $match: { department: "Development" } },
  { $project: { name: { $toLower: "$name.first" } } }
])