$trim (aggregation)¶

Definition定义¶

$trim¶

New in version 4.0.版本4.0中的新功能。

Removes whitespace characters, including null, or the specified characters from the beginning and end of a string.

$trim has the following syntax:语法如下所示：

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

The $trim takes a document with the following fields:

Field字段	Description描述
input	The string to trim. The argument can be any valid expression that resolves to a string. For more information on expressions, see Expressions.
chars	Optional.可选。The character(s) to trim from input. The argument can be any valid expression that resolves to a string. The $trim operator breaks down the string into individual UTF code point to trim from input. If unspecified, $trim removes whitespace characters, including the null character. For the list of whitespace characters, see Whitespace Characters.

See also参阅

$ltrim and $rtrim

Behavior行为¶

• By default, $trim removes whitespace characters, including the null character:

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

• You can override the default characters to trim using the chars field.

  For example, the following trims any g and e from the start and end of the input. Since the input starts with a whitespace, neither character can be trimmed from the start of the string.

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

• If overriding the default characters to trim, you can explicitly include the whitespace character(s) to trim in the chars field.

  For example, the following trims any space, g, e from the start and end of the input.

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

Example示例¶

Consider an inventory collection with the following documents:

{ "_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 $trim operator to remove leading and trailing whitespaces from the description field:

db.inventory.aggregate([
   { $project: { item: 1, description: { $trim: { input: "$description" } } } }
])

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

{ "_id" : 1, "item" : "ABC1", "description" : "product 1" }
{ "_id" : 3, "item" : "XYZ1", "description" : null }
{ "_id" : 2, "item" : "ABC2", "description" : "product 2 \n The product is in stock." }