$function (aggregation)

Schema Validation Restriction架构验证限制

You cannot use $function as part of schema validation query expression.不能将$function用作架构验证查询表达式的一部分。

Javascript Enablement支持Javascript

To use $function, you must have server-side scripting enabled (default).要使用$function，必须启用服务器端脚本（默认）。

If you do not use $function (or $accumulator, $where, or mapReduce), disable server-side scripting:如果不使用$function（或<$accumulator、$where或mapReduce），请禁用服务器端脚本：

• For a mongod instance, see security.javascriptEnabled configuration option or --noscripting command-line option.对于mongod实例，请参阅security.javascriptEnabled配置选项或--noscripting命令行选项。

• For a mongos instance, see security.javascriptEnabled configuration option or the --noscripting command-line option starting in MongoDB 4.4.对于mongos实例，请参阅security.javascriptEnabled配置选项或MongoDB 4.4中的--noscripting命令行选项。

  In earlier versions, MongoDB does not allow JavaScript execution on mongos instances.在早期版本中，MongoDB不允许在mongos实例上执行JavaScript。

See also ➤ Run MongoDB with Secure Configuration Options.另请参阅➤使用安全配置选项运行MongoDB。

Alternative to $where替代$where

The query operator $where can also be used to specify JavaScript expression. 查询运算符$where也可用于指定JavaScript表达式。However:然而：

• The $expr operator allows the use of aggregation expressions within the query language.$expr运算符允许在查询语言中使用聚合表达式。
• Starting in MongoDB 4.4, the $function and $accumulator allows users to define custom aggregation expressions in JavaScript if the provided pipeline operators cannot fulfill your application's needs.从MongoDB 4.4开始，如果提供的管道运算符不能满足应用程序的需要，$function和$accumulator允许用户用JavaScript定义自定义聚合表达式。

Given the available aggregation operators:给定可用的聚合运算符：

• The use of $expr with aggregation operators that do not use JavaScript (i.e. non-$function and non-$accumulator operators) is faster than $where because it does not execute JavaScript and should be preferred if possible.$expr与不使用JavaScript的聚合运算符（即非$function和非$accumulator运算符）一起使用要比$where快，因为它不执行JavaScript，如果可能的话应该优先使用。
• However, if you must create custom expressions, $function is preferred over $where.但是，如果必须创建自定义表达式，则首选$function而不是$where。

Example 1: Usage Example示例1：用法示例

Create a sample collection named players with the following documents:使用以下文档创建名为players的示例集合：

db.players.insertMany([
   { _id: 1, name: "Miss Cheevous",  scores: [ 10, 5, 10 ] },
   { _id: 2, name: "Miss Ann Thrope", scores: [ 10, 10, 10 ] },
   { _id: 3, name: "Mrs. Eppie Delta ", scores: [ 9, 8, 8 ] }
])

The following aggregation operation uses $addFields to add new fields to each document:以下聚合操作使用$addFields向每个文档添加新字段：

• isFound whose value is determined by the custom $function expression that checks whether the MD5 hash of the name is equal to a specified hash.其值由自定义$function表达式确定，该表达式检查名称的MD5哈希是否等于指定的哈希。
• message whose value is determined by the custom $function expression that format a string message using a template.其值由使用模板格式化字符串消息的自定义$function表达式确定。

db.players.aggregate( [
   { $addFields:
      {
        isFound:
            { $function:
               {
                  body: function(name) {
                     return hex_md5(name) == "15b0a220baa16331e8d80e15367677ad"
                  },
                  args: [ "$name" ],
                  lang: "js"
               }
            },
         message:
            { $function:
               {
                  body: function(name, scores) {
                     let total = Array.sum(scores);
                     return `Hello ${name}.  Your total score is ${total}.`
                  },
                  args: [ "$name", "$scores"],
                  lang: "js"
               }
            }
       }
    }
] )

The operation returns the following documents:该操作返回以下文档：

{ "_id" : 1, "name" : "Miss Cheevous", "scores" : [ 10, 5, 10 ], "isFound" : false, "message" : "Hello Miss Cheevous.  Your total score is 25." }
{ "_id" : 2, "name" : "Miss Ann Thrope", "scores" : [ 10, 10, 10 ], "isFound" : true, "message" : "Hello Miss Ann Thrope.  Your total score is 30." }
{ "_id" : 3, "name" : "Mrs. Eppie Delta ", "scores" : [ 9, 8, 8 ], "isFound" : false, "message" : "Hello Mrs. Eppie Delta .  Your total score is 25." }

Example 2: Alternative to $where示例2：替代$where

As an alternative to a query that uses the $where operator, you can use $expr and $function. 作为使用$where运算符的查询的替代方法，您可以使用$expr和$function。For example, consider the following $where example.例如，考虑下面的$where示例。

db.players.find( { $where: function() {
   return (hex_md5(this.name) == "15b0a220baa16331e8d80e15367677ad")
} } );

The db.collection.find() operation returns the following document:db.collection.find()操作返回以下文档：

{ "_id" : 2, "name" : "Miss Ann Thrope", "scores" : [ 10, 10, 10 ] }

The example can be expressed using $expr and $function:示例可以使用$expr和$function表示：

db.players.find( {$expr: { $function: {
      body: function(name) { return hex_md5(name) == "15b0a220baa16331e8d80e15367677ad"; },
      args: [ "$name" ],
      lang: "js"
} } } )