$literal (aggregation)

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Definition定义

$literal

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

The $literal expression has the following syntax:$literal表达式语法如下所示：

{ $literal: <value> }

Behavior行为

If the <value> is an expression, $literal does not evaluate the expression but instead returns the unparsed expression.如果<value>是表达式，$literal不计算表达式，而是返回未分析的表达式。

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

Examples示例

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；即提供进入现场的通道。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" ]在文档中名为price的字段中的值和名为1的字段中的值之间执行相等检查。

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

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

{ "_id" : 1, "item" : "abc123", price: "$2.50" }
{ "_id" : 2, "item" : "xyz123", price: "1" }
{ "_id" : 3, "item" : "ijk123", price: "$1" }
db.records.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":此操作会投影一个名为costsOneDollar的字段，该字段包含一个布尔值，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投影一个值为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<field>包含在输出中。The following example uses the $literal to return a new field set to the value of 1.下面的示例使用$literal返回一个设置为1的新字段。

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

{ "_id" : 1, "item" : "abc123", condition: "new" }
{ "_id" : 2, "item" : "xyz123", condition: "new" }

The following aggregation evaluates the expression item: 1 to mean return the existing field item in the output, but uses the { $literal: 1 } expression to return a new field startAt set to the value 1:以下聚合对表达式item: 1进行求值，表示返回输出中的现有字段项，但使用{ $literal: 1 }表达式将新字段startAt集返回到值1：

db.bids.aggregate( [
   { $project: { item: 1, startAt: { $literal: 1 } } }
] )

The operation results in the following documents:该操作将生成以下文档：

{ "_id" : 1, "item" : "abc123", "startAt" : 1 }
{ "_id" : 2, "item" : "xyz123", "startAt" : 1 }