$slice (projection)

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APPLIES TO: MongoDB vCore

The $slice operator is used to return a subset of an array. It can be used to limit the number of elements in an array to a specified number or to return elements from a specified position in the array. This operator is useful when dealing with large arrays where only a portion of the data is needed for processing or display.

Syntax

db.collection.find({},
  {
    <field>: { $slice: <count> }
  }
)

db.collection.find({},
  {
    <field>: { $slice: [ <skip>, <limit> ] }
  }
}

Parameters

Description
field The array field from which you want to slice a subset.
count The number of elements to return from the beginning of the array.
Description
skip The number of elements to skip.
limit The number of elements to return after skipping.

Examples

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: Returns the first matching element from an array

The example queries stores data for "Lakeshore Retail", and finds the first document from "sales.salesByCategory" array.

db.stores.find(
  { "name": "Lakeshore Retail"},
  { "_id":1,"name":1,"sales.salesByCategory": { $slice: 1 } } // restricts the fields to be returned
)

This query will return document from the stores collection, with first categoryName document within the array.

{
  "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
  "name": "Lakeshore Retail",
  "sales": {
    "salesByCategory": [
      {
        "categoryName": "Towel Racks",
        "totalSales": 13237
      }
    ]
  }
}

Example 2: Return the last element from an array

The example queries stores data for "Lakeshore Retail", and finds the last document from "sales.salesByCategory" array.

db.stores.find(
  { "name": "Lakeshore Retail"},
  { "_id":1,"name":1,"sales.salesByCategory": { $slice: -1 } } 
)

This query will return document from the stores collection, with last categoryName document within the array.

{
  "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
  "name": "Lakeshore Retail",
  "sales": {
    "salesByCategory": [
      {
        "categoryName": "Pillow Cases",
        "totalSales": 38833
      }
    ]
  }
}

Example 3: Returns a range of elements from an array

The example queries stores data for "Lakeshore Retail", and finds a subset range from "sales.salesByCategory" array.

db.stores.find(
  { "name": "Lakeshore Retail"},
  { "_id":1,"name":1,"sales.salesByCategory": { $slice: [3, 2] } }
)

This query returns document from the stores collection, with two elements starting from the fourth element of the sales.salesByCategory array.

{
  "_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
  "name": "Lakeshore Retail",
  "sales": {
    "salesByCategory": [
      {
        "categoryName": "Toothbrush Holders",
        "totalSales": 47912
      },
      {
        "categoryName": "Hybrid Mattresses",
        "totalSales": 48660
      }
    ]
  }
}

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