$concatArrays (array expression)

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APPLIES TO: MongoDB vCore

The $concatArrays operator is used to combine multiple arrays into a single array. This operator is useful when you need to merge arrays from different documents or fields in a document.

Syntax

The syntax for the $concatArrays operator is as follows:

{
  "$concatArrays": [ "<array1>", "<array2>"]
}

Parameters

Description
<array1>, <array2> The array fields targeted for concatenation.

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: Concatenating Arrays in a Document

The example pipeline merges the categoryName field from the promotionEvents.discounts array with the tag array into a single combinedTags array.

db.stores.aggregate([ 
        { $match: { _id: '7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5' } }
       ,{ $project: { combinedTags: { $concatArrays: ["$promotionEvents.discounts.categoryName", "$tag"] } } }
])

The query returns the arrays categoryName and tags merged as single array from the document.

{
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "combinedTags": [
      [ "DJ Turntables", "DJ Mixers" ],
      "#ShopLocal",
      "#FashionStore",
      "#SeasonalSale",
      "#FreeShipping",
      "#MembershipDeals"
    ]
  }

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