$toLower (aggregation)

Definition定义

$toLower

Converts a string to lowercase, returning the result.将字符串转换为小写，并返回结果。

$toLower has the following syntax:具有以下语法：

{ $toLower: <expression> }

The argument can be any expression as long as it resolves to a string. For more information on expressions, see Expressions.参数可以是任何表达式，只要它解析为字符串即可。有关表达式的详细信息，请参阅表达式。

If the argument resolves to null, $toLower returns an empty string "".如果参数解析为null，$toLower将返回一个空字符串""。

Behavior行为

$toLower only has a well-defined behavior for strings of ASCII characters.仅对ASCII字符字符串具有定义良好的行为。

Example实例

Consider a inventory collection with the following documents:考虑一个包含以下文档的inventory集合：

{ "_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 $toLower operator return lowercase item and lowercase description value:以下操作使用$toLower运算符返回小写item和小写description值：

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

The operation returns the following results:该操作返回以下结果：

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