$map (aggregation)¶

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Definition
Example

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`	The variable name for the items in the `input` array. The `in` expression accesses each item in the `input` array by this variable.
`in`	The expression to apply to each item in the `input` array. The expression accesses the item by its variable name.

For more information on expressions, see Expressions.

Example¶

Consider a grades collection with the following documents:

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

And the following $project statement:

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

The operation returns the following results:

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

参见

$let

← Aggregation Variable Operators $let (aggregation) →