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$bottomN (expression operator)

Definition

$bottomN: New in version 8.3.
Returns the last n elements of an array according to the specified sort order. If the array contains fewer than n elements, $bottomN returns all elements in the array.

Note

Disambiguation

This page describes the $bottomN expression operator. For the $bottomN accumulator operator, see $bottomN (accumulator operator).

Syntax

When used as an expression operator, $bottomN has the following syntax:

{
   $bottomN:
      {
         n: <expression>,
         sortBy: { <field1>: <sort order>, <field2>: <sort order> ... },
         input: <expression>
      }
}

Field	Necessity	Description
n	Required	The number of array elements to return.
sortBy	Required	Specifies the order of results. See Sort Behavior for more information.
input	Required	The array that `$bottomN` evaluates.

Behavior

Sort Behavior

MongoDB sorts the input array based on the sortBy value. The following table provides examples of different sort options:

input

sortBy

Sorted input

[ 8, 3, 1, 10]

1

[1, 3, 8, 10]

[ 8, 3, 1, 10]

-1

[10, 8, 3, 1]

[
  { a: 1, b: 1 },
  { a: 2, b: 2 },
  { a: 2, b: 1 }
]

{ a: 1, b: 1 }

[
  { a: 1, b: 1 },
  { a: 2, b: 1 },
  { a: 2, b: 2 }
]

Input Values

The input field must resolve to an array. If you specify an input that is not an array, MongoDB errors.

Example

The following example uses the movies collection from the sample_mflix sample dataset. For details on how to load this dataset into your deployment, see Load the sample dataset.

The movies collection contains documents that resemble the following example:

{
   _id: ObjectId('573a1396f29313caabce4a9a'),
   year: 1972,
   genres: [ 'Crime', 'Drama' ],
   title: 'The Godfather',
   cast: [ 'Marlon Brando', 'Al Pacino', 'James Caan', 'Richard S. Castellano' ],
   directors: [ 'Francis Ford Coppola' ],
   runtime: 175,
   ...
}

The following aggregation pipeline uses $bottomN on the cast array:

db.movies.aggregate([
   {
      $match: { title: "The Godfather" }
   },
   {
      $project: {
         _id: 0,
         title: 1,
         lastThreeCastMembersAlphabetically: {
            $bottomN: {
               n: 3,
               sortBy: 1,
               input: "$cast"
            }
         }
      }
   }
])

[
  {
    title: 'The Godfather',
    lastThreeCastMembersAlphabetically: [ 'James Caan', 'Marlon Brando', 'Richard S. Castellano' ]
  }
]

In this example, $bottomN sorts the existing cast array in ascending alphabetical order and returns the last three values.