$ltrim (aggregation)

Definition定义

$ltrim

Removes whitespace characters, including null, or the specified characters from the beginning of a string.删除字符串开头的空白字符，包括null或指定字符。

$ltrim has the following syntax:语法如下：

{ $ltrim: { input: <string>,  chars: <string> } }

The $ltrim takes a document with the following fields:$ltrim获取具有以下字段的文档：

Field字段	Description描述
input	The string to trim. 要修剪的字符串。The argument can be any valid expression that resolves to a string. 参数可以是解析为字符串的任何有效表达式。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。
chars	Optional.可选的。The character(s) to trim from the beginning of the input.从input开始要修剪的字符。 The argument can be any valid expression that resolves to a string. 参数可以是解析为字符串的任何有效表达式。The $ltrim operator breaks down the string into individual UTF code point to trim from input.$ltrim运算符将字符串分解为单独的UTF代码点，以便从输入中进行修剪。 If unspecified, $ltrim removes whitespace characters, including the null character. 如果未指定，$ltrim将删除空白字符，包括null字符。For the list of whitespace characters, see Whitespace Characters.有关空白字符的列表，请参阅空白字符。

Tip

See also: 另请参阅：

• $trim
• $rtrim

Behavior行为

• By default, $ltrim removes whitespace characters, including the null character, from the beginning of the input string:默认情况下，$ltrim会从输入字符串的开头删除空白字符，包括null字符：

  Example示例	Results结果
  { $ltrim: { input: " \n good bye \t " } }	"good bye \t "

• You can override the default characters to trim using the chars field.您可以使用chars字段覆盖要修剪的默认字符。

  For example, the following trims any g and e from the start of the input string. 例如，下面将从输入字符串的开头修剪任何g和e。Since the input starts with a whitespace, neither character can be trimmed from the start of the string.由于输入以空白开始，因此不能从字符串的开头对两个字符进行修剪。

  Example示例	Results结果
  { $ltrim: { input: " ggggoodbyeeeee", chars: "ge" } }	" ggggoodbyeeeee"

• If overriding the default characters to trim, you can explicitly include the whitespace character(s) to trim in the chars field.如果覆盖要修剪的默认字符，则可以在chars字段中显式包含要修剪的空白字符。

  For example, the following trims any space, g, or d from the start of the input string.例如，以下内容会修剪输入字符串开头的任何空格g或d。

  Example示例	Results结果
  { $ltrim: { input: " ggggoodbyeeeee ", chars: " gd" } }	"oodbyeeeee "

Example实例

Consider an 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 \n The product is in stock.  \n\n  " }
{ "_id" : 3, "item" : "XYZ1", quarter: "14Q2", "description" : null }

The following operation uses the $ltrim operator to remove leading whitespaces from the description field:以下操作使用$ltrim运算符从description字段中删除前导空格：

db.inventory.aggregate([
   { $project: { item: 1, description: { $ltrim: { input: "$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 \n The product is in stock.  \n\n  " }
{ "_id" : 3, "item" : "XYZ1", "description" : null }