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$literal (aggregation)

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Definition定义

$literal

Returns a value without parsing. Use for values that the aggregation pipeline may interpret as an expression.返回一个值而不进行分析。用于聚合管道可能解释为表达式的值。

The $literal expression has the following syntax:$literal表达式具有以下语法：

{ $literal: <value> }

Behavior行为

If the <value> is an expression, $literal does not evaluate the expression but instead returns the unparsed expression.如果<value>是一个表达式，$literal不会计算该表达式，而是返回未分析的表达式。

Example示例Result结果
{ $literal: { $add: [ 2, 3 ] } }{ "$add" : [ 2, 3 ] }
{ $literal: { $literal: 1 } }{ "$literal" : 1 }

Examples实例

Treat $ as a Literal将$视为文字

In expression, the dollar sign $ evaluates to a field path; i.e. provides access to the field. 在表达式中，美元符号$计算为字段路径；即提供对字段的访问。For example, the $eq expression $eq: ["$price", "$1" ] performs an equality check between the value in the field named price and the value in the field named 1 in the document.例如，$eq表达式$eq: ["$price", "$1" ]执行文档中名为price的字段中的值与名为1的字段中值之间的相等性检查。

The following example uses a $literal expression to treat a string that contains a dollar sign "$1" as a constant value.以下示例使用$literal表达式将包含美元符号"$1"的字符串视为常数值。

A storeInventory collection has the following documents:storeInventory集合包含以下文档：

db.storeInventory.insertMany( [
   { "_id" : 1, "item" : "napkins", price: "$2.50" },
   { "_id" : 2, "item" : "coffee", price: "1" },
   { "_id" : 3, "item" : "soap", price: "$1" }
] )
db.storeInventory.aggregate( [
   { $project: { costsOneDollar: { $eq: [ "$price", { $literal: "$1" } ] } } }
] )

This operation projects a field named costsOneDollar that holds a boolean value, indicating whether the value of the price field is equal to the string "$1":此操作投影一个名为costsOneDollar的字段，该字段包含布尔值，指示price字段的值是否等于字符串"$1"：

{ "_id" : 1, "costsOneDollar" : false }
{ "_id" : 2, "costsOneDollar" : false }
{ "_id" : 3, "costsOneDollar" : true }

Project a New Field with Value 1投影值为1的新字段

The $project stage uses the expression <field>: 1 to include the <field> in the output. $project阶段使用表达式<field>: 1将<field>包含在输出中。The following example uses the $literal to return a new field set to the value of 1.以下示例使用$literal返回一个值为1的新字段。

A books collection has the following documents:books集合包括以下文档：

{ "_id" : 1, "title" : "Dracula", "condition": "new" }
{ "_id" : 2, "title" : "The Little Prince", "condition": "new" }

The { $literal: 1 } expression returns a new editionNumber field set to the value 1:{ $literal: 1 }表达式返回一个新的editionNumber字段，该字段设置为值1:

db.books.aggregate( [
   { $project: { "title": 1, "editionNumber": { $literal: 1 } } }
] )

The operation results in the following documents:操作产生以下文件：

{ "_id" : 1, "title" : "Dracula", "editionNumber" : 1 }
{ "_id" : 2, "title" : "The Little Prince", "editionNumber" : 1 }