$filter

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

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

Parameters

inputstringrequired

An expression that resolves to an array.

asstringrequired

A string that specifies the variable name for each element in the input array.

condstringrequired

An expression that determines whether to include the element in the output array.

Examples

Sample Data

{
  "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
  "promotionEvents": [
    {
      "discounts": [
        { "categoryName": "DJ Turntables", "discountPercentage": 18 },
        { "categoryName": "DJ Mixers", "discountPercentage": 15 }
      ]
    }
  ]
}

Filter discounts above a threshold

Filter promotional discounts to only include those with discount percentage greater than 15.

Query:

db.stores.aggregate([
  { $match: { _id: "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5" } },
  {
    $project: {
      name: 1,
      highDiscounts: {
        $filter: {
          input: { $arrayElemAt: ["$promotionEvents.discounts", 0] },
          as: "discount",
          cond: { $gt: ["$$discount.discountPercentage", 15] }
        }
      }
    }
  }
])

Output:

[
  {
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "name": "Lakeshore Retail | DJ Equipment Stop",
    "highDiscounts": [
      { "categoryName": "DJ Turntables", "discountPercentage": 18 }
    ]
  }
]