How to Use $map in Aggregation in MongoDB

In MongoDB aggregation, use the $map operator to apply an expression to each element of an array and return a new array with the transformed values. It requires specifying the input array, a variable name for each element, and the transformation expression inside $map.

Syntax

The $map operator has three main parts:

• input: The array to transform.
• as: A variable name to represent each element in the array.
• in: The expression to apply to each element.

This creates a new array with each element replaced by the result of the expression.

json

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

Example

This example shows how to double each number in an array field called numbers using $map in an aggregation pipeline.

mongodb

db.collection.aggregate([
  {
    $project: {
      doubledNumbers: {
        $map: {
          input: "$numbers",
          as: "num",
          in: { $multiply: ["$$num", 2] }
        }
      }
    }
  }
])

Output

[
  { "doubledNumbers" : [2, 4, 6, 8] },
  { "doubledNumbers" : [10, 20, 30, 40] }
]

Common Pitfalls

Common mistakes when using $map include:

• Not using $$ before the variable name inside the in expression (e.g., use $$num not $num).
• Passing a non-array value to input, which causes errors.
• Forgetting to use $project or another stage to output the transformed array.

json

/* Wrong: missing $$ before variable */
{
  $map: {
    input: "$numbers",
    as: "num",
    in: { $multiply: ["$num", 2] }  // Incorrect
  }
}

/* Correct: use $$ before variable */
{
  $map: {
    input: "$numbers",
    as: "num",
    in: { $multiply: ["$$num", 2] }  // Correct
  }
}

Quick Reference

Part	Description	Example
input	Array to transform	"$numbers"
as	Variable name for each element	"num"
in	Expression applied to each element	{ $multiply: ["$$num", 2] }

Key Takeaways

Use $map to transform each element of an array in aggregation pipelines.

Always prefix the variable with $$ inside the in expression.

Ensure the input is an array to avoid errors.

Use $project or similar stages to output the transformed array.

The $map operator returns a new array with transformed elements.