$lastN (expression)

Definition

$lastN

Returns the specified number of elements from the end of an array.

Syntax

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

Behavior

• $lastN returns elements in the same order they appear in the input array.

• $lastN does not filter out null values in the input array.

• You cannot specify a value of n less than 1.

• If the specified n is greater than or equal to the number of elements in the input array, $lastN returns the entire input array.

• If input resolves to a non-array value, the expression errors.

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 the $lastN operator to return the last three cast members for each movie. The cast members are returned in the new field lastThreeCast created by $addFields.

db.movies.aggregate( [
   {
      $match: {
         title: { $in: [ "The Godfather", "Spirited Away", "The Dark Knight" ] }
      }
   },
   {
      $addFields: {
         lastThreeCast: { $lastN: { input: "$cast", n: 3 } }
      }
   },
   {
      $project: {
         _id: 0,
         title: 1,
         cast: 1,
         lastThreeCast: 1
      }
   },
   {
      $sort: { title: 1 }
   }
] )

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

The preceding pipeline:

• Uses $match to filter for three specific movies.

• Uses $addFields with $lastN to create a new field lastThreeCast containing the last three elements from the cast array.

• Uses $project to include only the title, cast, and lastThreeCast fields in the output.

• Uses $sort to sort the results by title.

Learn More

• $firstN

• $sortArray