$map

The $map operator is used to apply an expression to each element in an array and return an array with the applied results. This operator is useful for transforming arrays within documents, such as modifying each element or extracting specific fields.

Syntax

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

Parameters

Parameter Description
input The array processed by the expression.
as The variable name for each element in the array.
in The expression to apply to each element.

Example

Let's understand the usage with sample json from stores dataset.

{
  "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
  "name": "Lakeshore Retail",
  "location": {
    "lat": -51.3041,
    "lon": -166.0838
  },
  "staff": {
    "totalStaff": {
      "fullTime": 5,
      "partTime": 20
    }
  },
  "sales": {
    "totalSales": 266491,
    "salesByCategory": [
      {
        "categoryName": "Towel Racks",
        "totalSales": 13237
      },
      {
        "categoryName": "Washcloths",
        "totalSales": 44315
      },
      {
        "categoryName": "Face Towels",
        "totalSales": 42095
      },
      {
        "categoryName": "Toothbrush Holders",
        "totalSales": 47912
      },
      {
        "categoryName": "Hybrid Mattresses",
        "totalSales": 48660
      },
      {
        "categoryName": "Napkins",
        "totalSales": 31439
      },
      {
        "categoryName": "Pillow Cases",
        "totalSales": 38833
      }
    ]
  },
  "tag": [
    "#ShopLocal",
    "#FashionStore",
    "#SeasonalSale",
    "#FreeShipping",
    "#MembershipDeals"
  ]
}

Example 1: Extracting category names

The aggregation pipeline filters the stores collection for _id, then projects a new field categoryNames where each element in the salesByCategory array has its totalSales increased by 500 using the $map operator.

db.stores.aggregate([
  {$match: { "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60"}
   },
  {
    $project: {
      categoryNames: {
        $map: {
          input: "$sales.salesByCategory.totalSales",
          as: "category",
          in: {$add: ["$$category",500]}
        }
      }
    }
  }
])

The query returns the updated data for totalSales with 500 added to individual element from the sample json.

{
  "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
  "categoryNames": [
    13737,
    44815,
    42595,
    48412,
    49160,
    31939,
    39333
  ]
}

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