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$maxN (expression operator)

Definition

$maxN: Returns the n largest values in an array.

Note

Other Uses of $maxN

This page describes $maxN when used as an expression, which returns the largest n values from an array.

You can also use $maxN in these other contexts:

$maxN (accumulator), which returns the maximum n values within a group.
$maxN (window function), which is used in the $setWindowFields stage to return the largest n values from documents in a particular window.

Syntax

{ $maxN: { n: <expression>, input: <expression> } }

Field	Type	Description
`n`	Expression	Specifies the number of elements to return. Must resolve to a positive integer.
`input`	Expression	Specifies the input array from which to return the maximum `n` elements.

Behavior

$maxN filters out null values.
If the input array contains fewer than n elements, $maxN returns all elements in the array.
If input resolves to a non-array value, the operation returns an error.
If input contains both numeric and string elements, string elements are sorted before numeric elements according to the BSON comparison order.

Example

The examples on this page use data from the sample_mflix dataset. For details on how to load this dataset into your self-managed MongoDB deployment, see Load the Sample Dataset. If you made any modifications to the sample databases, you may need to drop and recreate the databases to run the examples on this page.

The following example uses $maxN to return the two alphabetically last cast members for each movie, stored in a new topTwoCast field created by $addFields.

db.movies.aggregate( [
   {
      $match: {
         title: { $in: [ "The Godfather", "Spirited Away", "The Dark Knight" ] }
      }
   },
   {
      $addFields: {
         topTwoCast: { $maxN: { n: 2, input: "$cast" } }
      }
   },
   {
      $project: {
         _id: 0,
         title: 1,
         cast: 1,
         topTwoCast: 1
      }
   },
   {
      $sort: { title: 1 }
   }
] )

[
  {
    title: 'Spirited Away',
    cast: [ 'Rumi Hiiragi', 'Miyu Irino', 'Mari Natsuki', 'Takashi Naitè' ],
    topTwoCast: [ 'Takashi Naitè', 'Rumi Hiiragi' ]
  },
  {
    title: 'The Dark Knight',
    cast: [ 'Christian Bale', 'Heath Ledger', 'Aaron Eckhart', 'Michael Caine' ],
    topTwoCast: [ 'Michael Caine', 'Heath Ledger' ]
  },
  {
    title: 'The Godfather',
    cast: [ 'Marlon Brando', 'Al Pacino', 'James Caan', 'Richard S. Castellano' ],
    topTwoCast: [ 'Richard S. Castellano', 'Marlon Brando' ]
  }
]

The preceding pipeline:

Uses $match to filter for three specific movies.
Uses $addFields with $maxN to create a new field topTwoCast containing the two alphabetically last values from the cast array with n: 2.
Uses $project to include only the title, cast, and topTwoCast fields in the output.
Uses $sort to sort the results by title.

Learn More

Aggregation Pipeline Limits

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$mergeObjects