$map (aggregation)¶

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Definition¶

$map¶

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

The $map expression has the following syntax:

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

Field	Specification
`input`	An expression that resolves to an array.
`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 Expressions.

Examples¶

Add to each element of an array using `$map`¶

A collection named grades has the following documents:

{ _id: 1, quizzes: [ 5, 6, 7 ] }
{ _id: 2, quizzes: [ ] }
{ _id: 3, quizzes: [ 3, 8, 9 ] }

The following aggregation operation outputs documents in which each member of the quizzes array is increased by 2.

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

This operation returns the following results:

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

Truncate each array element with `$map`¶

A collection named deliveries has the following documents:

{ "_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 the $trunc operator to truncate each member of the distances array to its integer value.

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

This operation returns the following results:

{ "_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 ] }

Pipeline with two `$map` operations¶

A collection named temperatures has the following documents:

{ "_id" : 1, "date" : "June 23", "temps" : [ 4, 12, 17 ] }
{ "_id" : 2, "date" : "July 7", "temps" : [ 14, 24, 11 ] }
{ "_id" : 3, "date" : "October 30", "temps" : [ 18, 6, 8 ] }

The following aggregation operation uses two $project stages to output each array of temperatures in Celsius as an array of temperatures in Fahrenheit. The first $project stage uses the _id: 0 construct to suppress the _id field in the output documents.

db.temperatures.aggregate(
   [
      { $project:
         {  _id: 0,
            date: "$date",
            tempsStep1:
               { $map:
                  {
                     input: "$temps",
                     as: "tempInCelsius",
                     in: { $multiply: [ "$$tempInCelsius", 9/5 ] }
                  }
               }
         }
      },
      { $project:
         {  date: "$date",
            "temps in Fahrenheit":
               { $map:
                  {
                     input: "$tempsStep1",
                     as: "tempStep1",
                     in: { $add: [ "$$tempStep1", 32 ] }
                  }
               }
         }
      }
   ]
)

This operation returns the following results:

{ "date" : "June 23", "temps in Fahrenheit" : [ 39.2, 53.6, 62.6 ] }
{ "date" : "July 7", "temps in Fahrenheit" : [ 57.2, 75.2, 51.8 ] }
{ "date" : "October 30", "temps in Fahrenheit" : [ 64.4, 42.8, 46.4 ] }

$map (aggregation)¶

Definition¶

Examples¶

Add to each element of an array using $map¶

Truncate each array element with $map¶

Pipeline with two $map operations¶

Add to each element of an array using `$map`¶

Truncate each array element with `$map`¶

Pipeline with two `$map` operations¶