$map (aggregation)

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Definition定义

$map

Applies an expression to each item in an array and returns an array with the applied results.将表达式应用于数组中的每个项，并返回一个包含应用结果的数组。

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

{ $map: { input: <expression>, as: <string>, in: <expression> } }
Field字段Specification规范文档
input An expression that resolves to an array.解析为数组的表达式。
as Optional.可选。A name for the variable that represents each individual element of the input array. 表示input数组中每个元素的变量的名称。If no name is specified, the variable name defaults to this.如果未指定名称，则变量名默认为this。
in An expression that is applied to each element of the input array. 应用于input数组的每个元素的表达式。The expression references each element individually with the variable name specified in as.表达式使用as中指定的变量名分别引用每个元素。

For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Examples示例

Add to each element of an array using $map使用$map添加到数组的每个元素

From the mongo shell, create a sample collection named grades with the following documents:在mongo shell中，使用以下文档创建名为grades的样本集合：

db.grades.insertMany([
  { _id: 1, quizzes: [ 5, 6, 7 ] },
  { _id: 2, quizzes: [ ] },
  { _id: 3, quizzes: [ 3, 8, 9 ] }
])

The following aggregation operation uses $map with the $add expression to increment each element in the quizzes array by 2.下面的聚合操作使用$map和$add表达式将quizzes数组中的每个元素增加2。

db.grades.aggregate(
   [
      { $project:
         { adjustedGrades:
            {
              $map:
                 {
                   input: "$quizzes",
                   as: "grade",
                   in: { $add: [ "$$grade", 2 ] }
                 }
            }
         }
      }
   ]
)

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

{ "_id" : 1, "adjustedGrades" : [ 7, 8, 9 ] }
{ "_id" : 2, "adjustedGrades" : [ ] }
{ "_id" : 3, "adjustedGrades" : [ 5, 10, 11 ] }

Truncate each array element with $map用$map截断每个数组元素

From the mongo shell, create a sample collection named deliveries with the following documents:从mongo shell创建一个名为deliveries的样本集合，其中包含以下文档：

db.deliveries.insertMany([
  { "_id" : 1, "city" : "Bakersfield", "distances" : [ 34.57, 81.96, 44.24 ] },
  { "_id" : 2, "city" : "Barstow", "distances" : [ 73.28, 9.67, 124.36 ] },
  { "_id" : 3, "city" : "San Bernadino", "distances" : [ 16.04, 3.25, 6.82 ] }
])

The following aggregation operation uses $map to truncate each element in the distances array to its integer.下面的聚合操作使用$map将distance数组中的每个元素截断为其整数。

db.deliveries.aggregate(
   [
      { $project:
         {  city: "$city",
            integerValues:
               { $map:
                  {
                     input: "$distances",
                     as: "decimalValue",
                     in: { $trunc: "$$decimalValue" }
                  }
            }
         }
      }
   ]
)

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

{ "_id" : 1, "city" : "Bakersfield", "integerValues" : [ 34, 81, 44 ] }
{ "_id" : 2, "city" : "Barstow", "integerValues" : [ 73, 9, 124 ] }
{ "_id" : 3, "city" : "San Bernadino", "integerValues" : [ 16, 3, 6 ] }

Convert Celsius Temperatures to Fahrenheit Using $map使用$map将摄氏温度转换为华氏温度

From the mongo shell, create a sample collection named temperatures with the following documents:在mongo shell中，创建一个名为temperatures的样本集合，其中包含以下文档：

db.temperatures.insertMany([
  { "_id" : 1, "date" : ISODate("2019-06-23"), "tempsC" : [ 4, 12, 17 ] },
  { "_id" : 2, "date" : ISODate("2019-07-07"), "tempsC" : [ 14, 24, 11 ] },
  { "_id" : 3, "date" : ISODate("2019-10-30"), "tempsC" : [ 18, 6, 8 ] }
])

The following aggregation operation uses the $addFields stage to add a new field to the documents called tempsF which contains Fahrenheit equivalents of the elements in the tempsC array. 下面的聚合操作使用$addFields阶段向名为tempsF的文档中添加一个新字段，其中包含tempsC数组中元素的华氏当量。To convert from Celsius to Fahrenheit, the operation uses $map to $multiply the Celsius values by 9/5 and then $add 32.要从摄氏度转换为华氏度，该操作使用$map将摄氏度值乘以9/5，然后再加上32。

 db.temperatures.aggregate( [
   { $addFields:
      {
         "tempsF":
            { $map:
               {
                  input: "$tempsC",
                  as: "tempInCelsius",
                  in: { $add: [ { $multiply: [ "$$tempInCelsius", 9/5 ] }, 32 ] }
               }
            }
       }
    }
] )

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

{ "_id" : 1, "date" : ISODate("2019-06-23T00:00:00Z"), "tempsC : [ 4, 12, 17 ], "tempsF" : [ 39.2, 53.6, 62.6 ] }
{ "_id" : 2, "date" : ISODate("2019-07-07T00:00:00Z"), "tempsC" : [ 14, 24, 11 ], "tempsF" : [ 57.2, 75.2, 51.8 ] }
{ "_id" : 3, "date" : ISODate("2019-10-30T00:00:00Z"), "tempsC" : [ 18, 6, 8 ], "tempsF" : [ 64.4, 42.8, 46.4 ] }

See also参阅