$toLower (aggregation)（聚合）

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. 参数可以是任何表达式，只要它解析为字符串即可。For more information on expressions, see Expressions.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字符串具有明确定义的行为。

Example示例

Consider a inventory collection with the following documents:考虑使用以下文件进行inventory集合：

db.inventory.insertMany( [
   { _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 $toLower operator return lowercase item and lowercase description value:以下操作使用$toLow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: "" }