$objectToArray (array expression operator)

APPLIES TO: MongoDB vCore

The $objectToArray operator is used to convert a document (that is, an object) into an array of key-value pairs. This operator can be useful when you need to manipulate or analyze the structure of a document in a more flexible array format.

The syntax for the $objectToArray operator is as follows:

{
  $objectToArray: <object>
}

Let's understand the usage with the following sample json.

{
  "_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
        }
      ]
    }
  ],
  "tag": [
    "#ShopLocal",
    "#SeasonalSale",
    "#FreeShipping",
    "#MembershipDeals"
  ]
}

Here are some examples of how to use the $objectToArray operator with the provided sample JSON.

The following aggregation pipeline converts the location field of the store document into an array of key-value pairs.

db.stores.aggregate([ 
    {
        $project: {
          locationArray: { $objectToArray: "$location" }
        }
    },
    // Limit the result to the first 3 documents
    { $limit: 3 }
])

This query would return the following document.

[
  {
    "_id": "649626c9-eda1-46c0-a27f-dcee19d97f41",
    "locationArray": [ { "k": "lat", "v": -85.0867 }, { "k": "lon", "v": -165.3524 } ]
  },
  {
    "_id": "8345de34-73ec-4a99-9cb6-a81f7b145c34",
    "locationArray": [ { "k": "lat", "v": -22.5751 }, { "k": "lon", "v": -12.4458 } ]
  },
  {
    "_id": "57cc4095-77d9-4345-af20-f8ead9ef0197",
    "locationArray": [ { "k": "lat", "v": -41.287 }, { "k": "lon", "v": -76.0176 } ]
  }
]

This example converts the totalStaff field of the staff document into an array of key-value pairs.

db.stores.aggregate([
    {
      $project: {
        staffArray: { $objectToArray: "$staff.totalStaff" }
      }
    },
    // Limit the result to the first 3 documents
    { $limit: 3 }
])

This query would return the following document.

[
  {
    "_id": "649626c9-eda1-46c0-a27f-dcee19d97f41",
    "staffArray": [ { "k": "fullTime", "v": 15 }, { "k": "partTime", "v": 9 } ]
  },
  {
    "_id": "8345de34-73ec-4a99-9cb6-a81f7b145c34",
    "staffArray": [ { "k": "fullTime", "v": 12 }, { "k": "partTime", "v": 14 } ]
  },
  {
    "_id": "57cc4095-77d9-4345-af20-f8ead9ef0197",
    "staffArray": [ { "k": "fullTime", "v": 4 }, { "k": "partTime", "v": 8 } ]
  }
]

This pipeline converts the promotionalDates field for each promotion event into an array of key-value pairs.

db.stores.aggregate([
    {
      $project: {
        promotionEvents: {
          $map: {
            input: "$promotionEvents",
            as: "event",
            in: {
              eventName: "$$event.eventName",
              promotionalDatesArray: { $objectToArray: "$$event.promotionalDates" }
            }
          }
        }
      }
    },
    // Limit the result to the first document
    { $limit: 1 }
])

This query would return the following document.

[
  {
    "_id": "649626c9-eda1-46c0-a27f-dcee19d97f41",
    "promotionEvents": [
      {
        "eventName": "Markdown Mayhem",
        "promotionalDatesArray": [
          { "k": "startDate", "v": { "Year": 2024, "Month": 3, "Day": 24 } },
          { "k": "endDate", "v": { "Year": 2024, "Month": 4, "Day": 3 } }
        ]
      },
      {
        "eventName": "Spectacular Savings Showcase",
        "promotionalDatesArray": [
          { "k": "startDate", "v": { "Year": 2024, "Month": 6, "Day": 22 } },
          { "k": "endDate", "v": { "Year": 2024, "Month": 7, "Day": 1 } }
        ]
      }
    ]
  }
]

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