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Aggregation Stages

$addFields (aggregation stage)

Definition

$addFields

Adds new fields to documents. $addFields outputs documents that contain all existing fields from the input documents and newly added fields.

The $addFields stage is equivalent to a $project stage that explicitly specifies all existing fields in the input documents and adds the new fields.

Note

You can also use the $set stage, which is an alias for $addFields.

Compatibility

You can use $addFields for deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Syntax

The stage has the following syntax:

{ $addFields: { <newField>: <expression>, ... } }

Specify the name of each field to add and set its value to an aggregation expression or an empty object. For more information on expressions, see Expressions.

Important

If the name of the new field is the same as an existing field name (including _id), $addFields overwrites the existing value of that field with the value of the specified expression.

Behavior

  • $addFields appends new fields to existing documents. You can include one or more $addFields stages in an aggregation operation.

  • $addFields accepts the embedding of objects where you can set a value to an aggregation expression or to an empty object. For example, the following nested objects are accepted:

    {$addFields: { a: { b: { } } } }

    To add a field or fields to embedded documents (including documents in arrays) use the dot notation. See example.

  • To add an element to an existing array field with $addFields, use with $concatArrays. See example.

Examples

The examples on this page use data from the sample_mflix sample 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.

Using Two $addFields Stages

The following operation uses two $addFields stages to first convert runtime to hours, then compute a licensing fee at $0.50 per hour:

db.movies.aggregate( [
   { $match: { runtime: { $gt: 1000 } } },
   {
      $addFields: {
         runtimeHours: {
            $floor: { $divide: [ "$runtime", 60 ] }
         },
         ratingOutOf100: { $multiply: [ "$imdb.rating", 10 ] }
      }
   },
   {
      $addFields: {
         licenseFeeUSD: {
            $multiply: [ "$runtimeHours", 0.50 ]
         }
      }
   }
] )
[
  { _id: ..., title: 'Baseball', runtime: 1140, runtimeHours: 19, licenseFeeUSD: 9.5 },
  { _id: ..., title: 'Centennial', runtime: 1256, runtimeHours: 20, licenseFeeUSD: 10 }
]
...

Adding Fields to an Embedded Document

Use dot notation to add new fields to embedded documents.

The following aggregation operation adds a certified field to the embedded imdb document in each movie:

db.movies.aggregate( [
   { $match: { runtime: { $gt: 1000 } } },
   { $addFields: { "imdb.certified": true } }
] )
[
  { _id: ..., title: 'Baseball', imdb: { certified: true, '...': '...' } },
  { _id: ..., title: 'Centennial', imdb: { certified: true, '...': '...' } }
]
...

Overwriting an Existing Field

Specifying an existing field name in an $addFields operation causes the original field to be replaced.

The following $addFields operation overwrites the runtime field to add 15 minutes:

db.movies.aggregate( [
   { $match: { runtime: { $gt: 1000 } } },
   {
      $addFields: { runtime: { $add: [ "$runtime", 15 ] } }
   }
] )
[
  { _id: ..., title: 'Baseball', runtime: 1155 },
  { _id: ..., title: 'Centennial', runtime: 1271 }
]
...

You can also replace one field with another. The following operation sets _id to the movie's title and replaces the title field with the movie's primary genre:

db.movies.aggregate( [
   { $match: { runtime: { $gt: 1000 } } },
   {
      $addFields: {
         _id: "$title",
         title: { $arrayElemAt: [ "$genres", 0 ] }
      }
   }
] )
[
  { _id: 'Baseball', title: 'Documentary' },
  { _id: 'Centennial', title: 'Action' }
]
...

Add Element to an Array

You can use $addFields with a $concatArrays expression to add an element to an existing array field. The following operation appends Epic to the genres array of movies with the title Centennial:

db.movies.aggregate( [
   { $match: { title: "Centennial" } },
   { $addFields: {
      genres: {
         $concatArrays: [ "$genres", [ "Epic" ] ]
      }
   } }
] )
[
  { _id: ..., title: 'Centennial', genres: [ 'Action', 'Adventure', 'Drama', 'Epic' ] }
]
...

Remove Fields

You can use $addFields with the $$REMOVE variable to remove document fields.

The following operation uses $addFields to remove the plot field with the $$REMOVE variable:

db.movies.aggregate( [
   { $match: { runtime: { $gt: 1000 } } },
   {
      $addFields: { plot: "$$REMOVE" }
   }
] )
[
  { _id: ..., title: 'Baseball', runtime: 1140 },
  { _id: ..., title: 'Centennial', runtime: 1256 }
]
...

You can also use $$REMOVE to conditionally remove fields. For example, the following aggregation removes the rated field from documents where rated is null:

db.movies.aggregate( [
   { $match: { runtime: { $gt: 1000 } } },
   {
      $addFields:
         {
            rated: {
               $ifNull: [ "$rated", "$$REMOVE" ]
            }
         }
   }
] )
[
  { _id: ..., title: 'Baseball', rated: 'TV-PG' },
  { _id: ..., title: 'Centennial' }
]
...

The Node.js examples on this page use the sample_mflix database from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started in the MongoDB Node.js driver documentation.

To use the MongoDB Node.js driver to add a $addFields stage to an aggregation pipeline, use the $addFields operator in a pipeline object.

The following example creates a pipeline stage that adds a totalReviews field to each movie document, which contains the movie's total number of reviews. The example then runs the aggregation pipeline:

const pipeline = [
  {
    $addFields: {
      totalReviews: {
        $add: ["$imdb.votes", "$tomatoes.viewer.numReviews"]
      }
    }
  }
];
const cursor = collection.aggregate(pipeline);
return cursor;

Tip

Comparison with $project

You can use either the $addFields or $project stage to remove document fields. The best approach depends on your pipeline and how much of the original document you want to retain.

For an example using $$REMOVE in a $project stage, see Conditionally Exclude Fields.

Learn More

To learn more about related pipeline stages, see the $project and $set guides.

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