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Window Functions

$addToSet (window function)

Definition

$addToSet

Returns an array of all unique values from a particular window.

The order of the elements in the returned array is unspecified.

Note

Disambiguation

This page describes $addToSet when used as a window function.

You can also use $addToSet in these other contexts:

Syntax

{
   $setWindowFields: {
      ...
      output: {
         <field>: {
            $addToSet: <expression>
         }
      }
   }
}

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.

This example uses the $addToSet window function to show how unique genres accumulate across movies over time. Movies are sorted alphabetically by title, and the window function tracks all unique genres that have appeared in movies from the beginning of the result set up to and including the current movie.

db.movies.aggregate( [
   {
      $match: {
         year: 2002,
         genres: { $exists: true, $ne: [] }
      }
   },
   {
      $group: {
         _id: { year: "$year", title: "$title", genre: { $first: "$genres" } },
      }
   },
   {
      $setWindowFields: {
         sortBy: { "_id.title": 1 },
         output: {
            genresSeenSoFar: {
               $addToSet: "$_id.genre",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   },
   {
      $project: {
         _id: 0,
         year: "$_id.year",
         title: "$_id.title",
         genre: "$_id.genre",
         genresSeenSoFar: 1,
         uniqueGenreCount: { $size: "$genresSeenSoFar" }
      }
   },
   {
      $limit: 8
   }
] )

In the example:

  • The $match stage filters to movies from 2002 with genre information.

  • The $group stage groups by year, title, and the first genre in the genres array.

  • The $setWindowFields stage uses $addToSet with an unbounded to current window to accumulate unique genres from the beginning of the result set up to the current document.

  • The $project stage reshapes the output and calculates uniqueGenreCount to show how the set size grows.

  • The $limit stage restricts output to 8 documents.

The output demonstrates how $addToSet accumulates unique genres across documents. Notice that genresSeenSoFar grows from 1 genre to 7 genres as each movie adds its genre to the set. The seventh document ('3 Extremes II') has the same uniqueGenreCount as the sixth because its genre (Horror) was already in the set. This demonstrates that $addToSet only adds unique values and ignores duplicates.

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