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Database Manual

Aggregation Operations

Reference

Operators

$map (aggregation)

Definition

$map: Applies an expression to each item in an array and returns an array with the applied results.

Compatibility

You can use $map for deployments hosted in the following environments:

MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

The $map expression has the following syntax:

{ $map: { input: <expression>, as: <string>, in: <expression> } }

Field	Specification
`input`	An expression that resolves to an array. If `input` resolves to `null` or refers to a missing field, `$map` returns `null`. If `input` resolves to a non-array, non-null value, the pipeline errors.
`as`	Optional. A name for the variable that represents each individual element of the `input` array. If no name is specified, the variable name defaults to `this`.
`in`	An expression that is applied to each element of the `input` array. The expression references each element individually with the variable name specified in `as`.

For more information on expressions, see Expression Operators.

Examples

Add to Each Element of an Array

In mongosh, create a sample collection named grades with the following documents:

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

The following aggregation operation uses $map with the $add expression to increment each element in the quizzes array by 2.

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

This operation returns the following results:

[
  {
    _id: ObjectId("6390b8f7237da390c6869a62"),
    adjustedGrades: [ 7, 8, 9 ]
  },
  {
    _id: ObjectId("6390b8f7237da390c6869a63"),
    adjustedGrades: []
  },
  {
    _id: ObjectId("6390b8f7237da390c6869a64"),
    adjustedGrades: [ 5, 10, 11 ]
  }
]

Truncate Each Array Element

In mongosh, create a sample collection named deliveries with the following documents:

db.deliveries.insertMany( [
  {
    "city" : "Bakersfield",
    "distances" : [ 34.57, 81.96, 44.24 ]
  },
  {
    "city" : "Barstow",
    "distances" : [ 73.28, 9.67, 124.36 ]
  },
  {
    "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.

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

This operation returns the following results:

[
  {
    _id: ObjectId("6390b9b1237da390c6869a65"),
    city: 'Bakersfield',
    integerValues: [ 34, 81, 44 ]
  },
  {
    _id: ObjectId("6390b9b1237da390c6869a66"),
    city: 'Barstow',
    integerValues: [ 73, 9, 124 ]
  },
  {
    _id: ObjectId("6390b9b1237da390c6869a67"),
    city: 'San Bernadino',
    integerValues: [ 16, 3, 6 ]
  }
]

Convert Celsius Temperatures to Fahrenheit

In mongosh, create a sample collection named temperatures with the following documents:

db.temperatures.insertMany( [
  {
    "date" : ISODate("2019-06-23"),
    "tempsC" : [ 4, 12, 17 ]
  },
  {
    "date" : ISODate("2019-07-07"),
    "tempsC" : [ 14, 24, 11 ]
  },
  {
    "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. To convert from Celsius to Fahrenheit, the operation uses $map to $multiply the Celsius values by 9/5 and then $add 32.

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

This operation returns the following results:

[
  {
    _id: ObjectId("6390ba11237da390c6869a68"),
    date: ISODate("2019-06-23T00:00:00.000Z"),
    tempsC: [ 4, 12, 17 ],
    tempsF: [ 39.2, 53.6, 62.6 ]
  },
  {
    _id: ObjectId("6390ba11237da390c6869a69"),
    date: ISODate("2019-07-07T00:00:00.000Z"),
    tempsC: [ 14, 24, 11 ],
    tempsF: [ 57.2, 75.2, 51.8 ]
  },
  {
    _id: ObjectId("6390ba11237da390c6869a6a"),
    date: ISODate("2019-10-30T00:00:00.000Z"),
    tempsC: [ 18, 6, 8 ],
    tempsF: [ 64.4, 42.8, 46.4 ]
  }
]

Learn More

To learn more about expressions used in the previous examples, see:

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$max