$arrayToObject

The $arrayToObject operator is used to convert an array into a single document. This operator is useful when you need to transform arrays of key-value pairs into a more structured document format.

Syntax

{
  $arrayToObject: "<array>"
}

Parameters

Parameter Description
<array> The array to be converted into a document. Each element in the array must be either: a) A two-element array where the first element is the field name and the second element is the field value. b) A document with exactly two fields, "k" and "v", where "k" is the field name and "v" is the field value.

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 },
      { "categoryName": "DJ Cables", "totalSales": 1000 }
    ],
    "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: Convert the array to a key: value document

The example aggregation query converts the salesByCategory array into an object where each categoryName is a key and totalSales is the corresponding value. This transformation simplifies access to sales data by category directly from an object structure.

db.stores.aggregate([
  { $match: { _id: '7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5' } },
  { $project: { "sales.salesByCategory": { $arrayToObject: { $map: { input: "$sales.salesByCategory", as: "item", in: { k: "$$item.categoryName", v: "$$item.totalSales" } } } } } }
])

The query returns the converted salesByCategory array as a document.

{
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "sales": { "salesByCategory": { "DJ Headphones": 35921, "DJ Cables": 1000 } }
}

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