$reduce (array expression)

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APPLIES TO: MongoDB vCore

The $reduce operator is used to apply an expression to each element in an array and accumulate the results into a single value. This operator is particularly useful for performing operations that require iterating over array elements and aggregating their values.

Syntax

$reduce: {
   input: <array>,
   initialValue: <expression>,
   in: <expression>
}

Parameters

Description
input The array to iterate over.
initialValue The initial cumulative value set before the array iteration begins.
in A valid expression that evaluates to the accumulated value for each element in the array.

Example

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

{
  "_id": "d3c9df51-41bd-4b4e-a26b-b038d9cf8b45",
  "location": {
    "lat": -67.7571,
    "lon": 97.2505
  },
  "sales": {
    "totalSales": 149849,
    "salesByCategory": [
      {
        "categoryName": "DJ Speakers",
        "totalSales": 36972
      },
      {
        "categoryName": "DJ Headphones",
        "totalSales": 12877
      },
      {
        "categoryName": "Music Accessories",
        "totalSales": 40000
      },
      {
        "categoryName": "DJ Accessories",
        "totalSales": 60000
      }
    ]
  },
  "tag": [
    "#ShopLocal",
    "#FashionStore",
    "#SeasonalSale",
    "#FreeShipping",
    "#MembershipDeals"
  ]
}

Example 1: Aggregates the array values

The example demonstrates how to use $reduce to sum the total sales across different categories in the salesByCategory array.

db.stores.aggregate([
  { $match: { "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60"} }
, {
    $project: {
      totalSalesByCategory: {
        $reduce: {
          input: "$sales.salesByCategory.totalSales",
          initialValue: 0,
          in: { $add: ["$$value", "$$this"] }
        }
      }
    }
  }
])

The query returns totalSales across the categories.

{
    "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
    "totalSalesByCategory": 149849
}

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