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$filter (array expression)

APPLIES TO: MongoDB vCore

The $filter operator is used to filter elements from an array based on a specified condition. This operator is useful when you need to manipulate or retrieve specific array elements within documents.

Syntax

The basic syntax of the $filter operator is as follows:

{
  "$filter": {
    "input": "<array>",
    "as": "<string>",
    "cond": "<expression>"
  }
}

Parameters

Description
input An expression that resolves to an array.
as A string that specifies the variable name for each element in the input array.
cond An expression that determines whether to include the element in the output array.

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: Retrieve an element filtered on condition

The following example demonstrates how to filter sales category based on totalSales.

db.yourCollection.aggregate([
  { $match: { _id: '7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5' } },
  { $project: { filteredSalesByCategory: { $filter: { input: "$sales.salesByCategory", as: "item", cond: { $gt: ["$$item.totalSales", 10000] } } } } }
])

The resulting document has an extra field, filteredSalesByCategory, which include the items from salesByCategory that have totalSales greater than 10000.

{
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "filteredSalesByCategory": [ { "categoryName": "DJ Headphones", "totalSales": 35921 } ]
}

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