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$mergeObjects (accumulator operator)

Definition

$mergeObjects

Returns a document created by combining the input documents for each group.

The $mergeObjects accumulator is available in these stages:

Note

Disambiguation

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

You can also use $mergeObjects in these other contexts:

$mergeObjects (expression), which merges an array of documents into a single document.
$mergeObjects (window function), which is used in the $setWindowFields stage to return a merged document from documents in a window.

Syntax

{ $mergeObjects: <expression> }

The <expression> can be any valid expression that evaluates to a document.

Behavior

$mergeObjects ignores null input values. If all input values are null, $mergeObjects returns an empty document { }.

$mergeObjects overwrites field values as it merges documents. If multiple input documents contain the same field name, the output contains the value from the last input document that contains the conflicting field.

Example

Create a movieReviews collection:

db.movieReviews.insertMany( [
   {
      title: "Galactic Odyssey",
      source: "filmCritic",
      data: { rating: 8.1, votes: 1482903 }
   },
   {
      title: "Galactic Odyssey",
      source: "viewerVault",
      data: { tomatometer: 79, audienceScore: 85 }
   },
   {
      title: "Neon Horizon",
      source: "filmCritic",
      data: { rating: 7.4, votes: 983201 }
   },
   {
      title: "Neon Horizon",
      source: "viewerVault",
      data: { tomatometer: 68, audienceScore: 74 }
   },
   {
      title: "The Last Protocol",
      source: "filmCritic",
      data: { rating: 8.7, votes: 2104567 }
   },
   {
      title: "The Last Protocol",
      source: "viewerVault",
      data: { tomatometer: 91, audienceScore: 88 }
   }
] )

Each movie has two documents:

One from filmCritic with rating and votes fields.
One from viewerVault with tomatometer and audienceScore fields.

Because the two source documents have different field names, $mergeObjects can combine them into a complete profile that includes all four fields.

The following pipeline uses $mergeObjects in a $group stage to combine the review data from each source into a single document for each movie:

db.movieReviews.aggregate( [
   {
      $group: {
         _id: "$title",
         allRatings: { $mergeObjects: "$data" }
      }
   },
   { $sort: { _id: 1 } }
] )

[
  {
    _id: 'Galactic Odyssey',
    allRatings: {
      rating: 8.1,
      votes: 1482903,
      tomatometer: 79,
      audienceScore: 85
    }
  },
  {
    _id: 'Neon Horizon',
    allRatings: {
      rating: 7.4,
      votes: 983201,
      tomatometer: 68,
      audienceScore: 74
    }
  },
  {
    _id: 'The Last Protocol',
    allRatings: {
      rating: 8.7,
      votes: 2104567,
      tomatometer: 91,
      audienceScore: 88
    }
  }
]

In the preceding pipeline:

$group groups documents by title and uses $mergeObjects to merge the data sub-documents from each source. Because the FilmCritic and ViewerVault documents contain different field names, the merged result includes all fields from both sources.
$sort sorts the results alphabetically by title.

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