Aggregation Stages

$addFields

Adds new fields to documents. Similar to $project, $addFields reshapes each document in the stream; specifically, by adding new fields to output documents that contain both the existing fields from the input documents and the newly added fields.

$set is an alias for $addFields.

$bucket

Categorizes incoming documents into groups, called buckets, based on a specified expression and bucket boundaries.

$bucketAuto

Categorizes incoming documents into a specific number of groups, called buckets, based on a specified expression. Bucket boundaries are automatically determined in an attempt to evenly distribute the documents into the specified number of buckets.

$count

Returns a count of the number of documents at this stage of the aggregation pipeline.

Distinct from the $count aggregation accumulator.

$densify

Creates new documents in a sequence of documents where certain values in a field are missing.

$documents

Returns literal documents from input expressions.

$facet

Processes multiple aggregation pipelines within a single stage on the same set of input documents. Enables the creation of multi-faceted aggregations capable of characterizing data across multiple dimensions, or facets, in a single stage.

$fill

Populates null and missing field values within documents.

$geoNear

Returns an ordered stream of documents based on the proximity to a geospatial point. Incorporates the functionality of $match, $sort, and $limit for geospatial data. The output documents include an additional distance field and can include a location identifier field.

$graphLookup

Performs a recursive search on a collection. To each output document, adds a new array field that contains the traversal results of the recursive search for that document.

$group

Groups input documents by a specified identifier expression and applies the accumulator expression(s), if specified, to each group. Consumes all input documents and outputs one document per each distinct group. The output documents only contain the identifier field and, if specified, accumulated fields.

$limit

Passes the first n documents unmodified to the pipeline where n is the specified limit. For each input document, outputs either one document (for the first n documents) or zero documents (after the first n documents).

$lookup

Performs a left outer join to another collection or view in the same database to filter in documents from the "joined" collection or view for processing.

$match

Filters the document stream to allow only matching documents to pass unmodified into the next pipeline stage. $match uses standard MongoDB queries. For each input document, outputs either one document (a match) or zero documents (no match).

$merge

Writes the resulting documents of the aggregation pipeline to a collection. The stage can incorporate (insert new documents, merge documents, replace documents, keep existing documents, fail the operation, process documents with a custom update pipeline) the results into an output collection. To use the $merge stage, it must be the last stage in the pipeline.

$out

Writes the resulting documents of the aggregation pipeline to a collection. To use the $out stage, it must be the last stage in the pipeline.

$project

Reshapes each document in the stream, such as by adding new fields or removing existing fields. For each input document, outputs one document.

See also $unset for removing existing fields.

$redact

Reshapes each document in the stream by restricting the content for each document based on information stored in the documents themselves. Incorporates the functionality of $project and $match. Can be used to implement field level redaction. For each input document, outputs either one or zero documents.

$replaceRoot

Replaces a document with the specified embedded document. The operation replaces all existing fields in the input document, including the _id field. Specify a document embedded in the input document to promote the embedded document to the top level.

$replaceWith is an alias for $replaceRoot stage.

$replaceWith

Replaces a document with the specified embedded document. The operation replaces all existing fields in the input document, including the _id field. Specify a document embedded in the input document to promote the embedded document to the top level.

$replaceWith is an alias for $replaceRoot stage.

$sample

Randomly selects the specified number of documents from its input.

$set

Adds new fields to documents. Similar to $project, $set reshapes each document in the stream; specifically, by adding new fields to output documents that contain both the existing fields from the input documents and the newly added fields.

$set is an alias for $addFields stage.

$setWindowFields

Groups documents into windows and applies one or more operators to the documents in each window.

$skip

Skips the first n documents where n is the specified skip number and passes the remaining documents unmodified to the pipeline. For each input document, outputs either zero documents (for the first n documents) or one document (if after the first n documents).

$sort

Reorders the document stream by a specified sort key. Only the order changes; the documents remain unmodified. For each input document, outputs one document.

$sortByCount

Groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.

$unionWith

Performs a union of two collections; i.e. combines pipeline results from two collections or views into a single result set.

$unset

Removes/excludes fields from documents.

$unset is an alias for $project stage that removes fields.

$unwind

Deconstructs an array field from the input documents to output a document for each element. Each output document replaces the array with an element value. For each input document, outputs n documents where n is the number of array elements and can be zero for an empty array.

$changeStream

Returns a Change Stream cursor for the collection. This stage can only occur once in an aggregation pipeline and it must occur as the first stage.

$changeStreamSplitLargeEvent

Splits large change stream events that exceed 16 MB into smaller fragments returned in a change stream cursor.

You can only use $changeStreamSplitLargeEvent in a $changeStream pipeline and it must be the final stage in the pipeline.

$rankFusion

Combines the results of input pipelines that rank documents. $rankFusion de-duplicates input pipeline results and uses the Reciprocal Rank Fusion algorithm to create a final document ranking.

$search

Performs a full-text search of the field or fields in a collection.

To learn more, see MongoDB Search Aggregation Pipeline Stages.

$searchMeta

Returns different types of metadata result documents for the MongoDB Search query against an Atlas collection. To learn more, see MongoDB Search Aggregation Pipeline Stages.

$vectorSearch

Performs an ANN or ENN search on a vector in the specified field of an Atlas collection.

$vectorSearch is only available for MongoDB Atlas clusters running MongoDB v6.0.11 or higher, and is not available for self-managed deployments.

$collStats

Returns statistics regarding a collection or view.

$currentOp

Returns information on active and/or dormant operations for the MongoDB deployment.

Uses the db.aggregate() method instead of the db.collection.aggregate() method.

$indexStats

Returns statistics regarding the use of each index for the collection.

$listClusterCatalog

Retrieves information for collections in a cluster, including names and creation options.

$listLocalSessions

Lists all active sessions recently in use on the currently connected mongos or mongod instance. These sessions may have not yet propagated to the system.sessions collection.

Uses the db.aggregate() method instead of the db.collection.aggregate() method.

$listSampledQueries

Lists sampled queries for all collections or a specific collection.

$listSearchIndexes

Returns information about existing MongoDB Search indexes on a specified collection or view.

$listSessions

Lists all sessions that have been active long enough to propagate to the system.sessions collection.

$planCacheStats

Returns plan cache information for a collection.

$querySettings

Returns query settings previously added with setQuerySettings.

$queryStats

Returns runtime statistics for recorded queries.

WARNING: The $queryStats aggregation stage is unsupported and is not guaranteed to be stable in a future release. Don't build functionality that relies on a specific output format of this stage, since the output may change in a future release.

$shardedDataDistribution

Returns information on the distribution of data in sharded collections.

Uses the db.aggregate() method instead of the db.collection.aggregate() method.