$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.
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. |
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
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: