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$addToSet (accumulator operator)

Definition

$addToSet

Returns an array of unique values for an expression evaluated across documents in a group. The order of elements in the array is unspecified.

$addToSet is available in these stages:

Note

Disambiguation

This page describes $addToSet when used as an accumulator. Accumulators return a single aggregated value like sum, maximum, or minimum across a group of input documents.

You can also use $addToSet in these other contexts:

$addToSet (window function), which is used in the $setWindowFields stage to return an array of unique values from documents in a particular window.
$addToSet (update operator), which adds unique elements to an array field in a document.

Syntax

{ $addToSet: <expression> }

For more information on expressions, see Expressions.

Behavior

If the value of the expression is an array, $addToSet appends the whole array as a single element.

If the value of the expression is a document, MongoDB determines that the document is a duplicate if another document in the array matches the to-be-added document exactly. Specifically, the existing document has the exact same fields and values in the exact same 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 operation groups movies by director and uses the $addToSet accumulator to collect the unique genres each director has worked in. To improve readability, the pipeline limits the results to the first five directors in alphabetical order.

db.movies.aggregate( [
   {
      $unwind: "$directors"
   },
   {
      $unwind: "$genres"
   },
   {
      $group: {
         _id: "$directors",
         genres: { $addToSet: "$genres" }
      }
   },
   {
      $sort: { _id: 1 }
   },
   {
      $limit: 5
   }
] )

This pipeline returns these results:

[
  { _id: 'A. Dean Bell', genres: [ 'Crime', 'Drama' ] },
  { _id: 'A. Karunakaran', genres: [ 'Romance' ] },
  { _id: 'A. Vanichkin', genres: [ 'Animation', 'Fantasy' ] },
  { _id: 'A.J. Edwards', genres: [ 'Biography', 'Drama', 'History' ] },
  {
    _id: 'A.R. Murugadoss',
    genres: [ 'Drama', 'Sci-Fi', 'Thriller', 'Action', 'Mystery' ]
  }
]

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$accumulator

$avg