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$concatArrays (accumulator operator)

Definition

$concatArrays

Returns a single array that concatenates the array values of a field across a group of documents.

The $concatArrays accumulator is available in these stages:

Note

Disambiguation

This page describes $concatArrays when used as an accumulator. Accumulators return a single aggregated value across a group of input documents.

You can also use $concatArrays in these other contexts:

$concatArrays (expression), which concatenates two or more arrays into a single array within a single document.
$concatArrays (window function), which is used in the $setWindowFields stage to concatenate the array values from a field across a window of documents.

Syntax

{ $concatArrays: [ <expression1>, <expression2>, ... ] }

Behavior

Arguments that are null or evaluate to null are included in the results. Arguments that refer to a field that is missing are not included in the results.

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 example uses $concatArrays to concatenate the genres arrays of movies grouped by release year:

db.movies.aggregate( [
   {
      $match: { year: { $in: [1928, 1929] } }
   },
   {
      $group: {
         _id: "$year",
         combinedGenres: { $concatArrays: "$genres" }
      }
   },
   { $sort: { _id: 1 } }
] )

[
  {
    _id: 1928,
    combinedGenres: [
      'Drama',   'War',
      'Drama',   'Drama',
      'History', 'War',
      'Drama',   'Animation',
      'Family',  'Comedy',
      'Drama',   'Romance',
      'Western', 'Short',
      'Drama'
    ]
  },
  {
    _id: 1929,
    combinedGenres: [
      'Drama',   'Musical',
      'Drama',   'Musical',
      'Romance', 'Biography',
      'Drama',   'History',
      'Drama',   'Musical',
      'Drama',   'Comedy',
      'Short'
    ]
  }
]

The operation uses $match to select movies released in 1928 and 1929, then groups the results by year. The combinedGenres field concatenates the genres arrays of all movies in each year group.

Limitations

$concatArrays only supports arrays and expressions that resolve to an array.

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