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$toLower (aggregation)

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Definition

$toLower

Converts a string to lowercase, returning the result.

$toLower has the following syntax:

{ $toLower: <expression> }

The argument can be any expression as long as it resolves to a string. For more information on expressions, see Expressions.

If the argument resolves to null, $toLower returns an empty string "".

Behavior

$toLower only has a well-defined behavior for strings of ASCII characters.

Example

Consider a inventory collection with the following documents:

{ "_id" : 1, "item" : "ABC1", quarter: "13Q1", "description" : "PRODUCT 1" }
{ "_id" : 2, "item" : "abc2", quarter: "13Q4", "description" : "Product 2" }
{ "_id" : 3, "item" : "xyz1", quarter: "14Q2", "description" : null }

The following operation uses the $toLower operator return lowercase item and lowercase description value:

db.inventory.aggregate(
   [
     {
       $project:
         {
           item: { $toLower: "$item" },
           description: { $toLower: "$description" }
         }
     }
   ]
)

The operation returns the following results:

{ "_id" : 1, "item" : "abc1", "description" : "product 1" }
{ "_id" : 2, "item" : "abc2", "description" : "product 2" }
{ "_id" : 3, "item" : "xyz1", "description" : "" }