$size (array expression)

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APPLIES TO: MongoDB vCore

The $size operator is used to count the number of elements in an array. It is particularly useful when you need to determine the size of an array field within a document. This operator can be used in aggregation pipelines and in query projections.

Syntax

The syntax for using the $size operator is as follows:

{ $size: <array> }

Parameters

Description
array The array whose number of elements you want to count.

Example

Let's understand the usage with sample json from stores dataset.

{
  "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
  "name": "Lakeshore Retail",
  "sales": {
    "salesByCategory": [
      {
        "categoryName": "Towel Racks",
        "totalSales": 13237
      },
      {
        "categoryName": "Washcloths",
        "totalSales": 44315
      },
      {
        "categoryName": "Face Towels",
        "totalSales": 42095
      },
      {
        "categoryName": "Toothbrush Holders",
        "totalSales": 47912
      },
      {
        "categoryName": "Hybrid Mattresses",
        "totalSales": 48660
      },
      {
        "categoryName": "Napkins",
        "totalSales": 31439
      },
      {
        "categoryName": "Pillow Cases",
        "totalSales": 38833
      }
    ]},
"tag": [
    '#ShopLocal',
    '#FashionStore',
    '#SeasonalSale',
    '#FreeShipping',
    '#MembershipDeals'
  ]
}

Example 1: Find number of elements in an array

The example query calculates and returns the count of elements in the salesByCategory array for the provided JSON sample.

db.stores.aggregate([
  {
    $match: { "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60"}
  },
  {
    $project: {
      "storeId": 1,
      "name": 1,
      "numberOfSalesCategories": { $size: "$sales.salesByCategory" }
    }
  }
])

The query returns the number of sales categories available for the "Lakeshore Retail" store.

{
  "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
  "name": "Lakeshore Retail",
  "numberOfSalesCategories": 7
}

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