$reverseArray

The $reverseArray operator is used to reverse the order of elements in an array. This operator can be useful when you need to process or display array elements in the opposite order. It's an array expression operator and can be used within aggregation pipelines.

Syntax

{
  $reverseArray: <array>
}

Parameters

Parameter Description
<array> The array that you want to reverse.

Example

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

{
  "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
  "name": "Lakeshore Retail | DJ Equipment Stop - Port Cecile",
  "location": {
    "lat": 60.1441,
    "lon": -141.5012
  },
  "staff": {
    "totalStaff": {
      "fullTime": 2,
      "partTime": 0
    }
  },
  "sales": {
    "salesByCategory": [
      {
        "categoryName": "DJ Headphones",
        "totalSales": 35921
      }
    ],
    "fullSales": 3700
  },
  "promotionEvents": [
    {
      "eventName": "Bargain Blitz Days",
      "promotionalDates": {
        "startDate": {
          "Year": 2024,
          "Month": 3,
          "Day": 11
        },
        "endDate": {
          "Year": 2024,
          "Month": 2,
          "Day": 18
        }
      },
      "discounts": [
        {
          "categoryName": "DJ Turntables",
          "discountPercentage": 18
        },
        {
          "categoryName": "DJ Mixers",
          "discountPercentage": 15
        }
      ]
    },
    {
      "eventName": "Discount Delight Days",
      "promotionalDates": {
        "startDate": {
          "Year": 2024,
          "Month": 5,
          "Day": 11
        },
        "endDate": {
          "Year": 2024,
          "Month": 5,
          "Day": 18
        }
      }
    }
  ],
  "tag": [
    "#ShopLocal",
    "#FashionStore",
    "#SeasonalSale",
    "#FreeShipping",
    "#MembershipDeals"
  ]
}

Example 1: Reversing the order of an array

The aggregation pipeline demonstrates the usage of operator for performing reversal on the order of the promotionEvents array.

db.stores.aggregate([
    //filtering to one document
    {
      $match: {
        "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5"
      }
    },
    {
      $project: {
        _id: 1,
        name: 1,
        promotionEventsReversed: { $reverseArray: "$promotionEvents" }
      }
    },
    // Include only _id, name, promotionalDates and eventName fields in the output 
    { $project: { _id: 1, name: 1 , "promotionEventsReversed.promotionalDates":1, "promotionEventsReversed.eventName":1  } } 
])

The query returns document with reversed order for promotionEvents.

  {
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "name": "Lakeshore Retail | DJ Equipment Stop - Port Cecile",
    "promotionEventsReversed": [
      {
        "eventName": "Discount Delight Days",
        "promotionalDates": {
          "startDate": { "Year": 2024, "Month": 5, "Day": 11 },
          "endDate": { "Year": 2024, "Month": 5, "Day": 18 }
        }
      },
      {
        "eventName": "Bargain Blitz Days",
        "promotionalDates": {
          "startDate": { "Year": 2024, "Month": 3, "Day": 11 },
          "endDate": { "Year": 2024, "Month": 2, "Day": 18 }
        }
      }
    ]
  }

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