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Aggregation Pipeline Stages

Aggregation Pipeline Stages¶

In the db.collection.aggregate method and db.aggregate method, pipeline stages appear in an array. Documents pass through the stages in sequence.

Stages¶

`db.collection.aggregate()` Stages¶

All except the $out, $merge, and $geoNear stages can appear multiple times in a pipeline.

Note

For details on specific operator, including syntax and examples, click on the specific operator to go to its reference page.

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db.collection.aggregate( [ { <stage> }, ... ] )

Stage	Description
`$addFields`	Adds new fields to documents. Similar to `$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.
`$bucketAuto`	Categorizes incoming documents into a specific number of groups, called buckets, based on a specified expression. Bucket boundaries are automatically determined in an attempt to evenly distribute the documents into the specified number of buckets.
`$collStats`	Returns statistics regarding a collection or view.
`$count`	Returns a count of the number of documents at this stage of the aggregation pipeline.
`$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.
`$geoNear`	Returns an ordered stream of documents based on the proximity to a geospatial point. Incorporates the functionality of `$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.
`$indexStats`	Returns statistics regarding the use of each index for the collection.
`$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).
`$listSessions`	Lists all sessions that have been active long enough to propagate to the `system.sessions` collection.
`$lookup`	Performs a left outer join to another collection in the same database to filter in documents from the “joined” collection 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. New in version 4.2.
`$out`	Writes the resulting documents of the aggregation pipeline to a collection. To use the `$out` stage, it must be the last stage in the pipeline.
`$planCacheStats`	Returns plan cache information for a collection.
`$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.
`$skip`	Skips the first n documents where n is the specified skip number and passes the remaining documents unmodified to the pipeline. For each input document, outputs either zero documents (for the first n documents) or one document (if after the first n documents).
`$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 into a single result set. New in version 4.4.
`$unset`	Removes/excludes fields from documents. `$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.

For aggregation expression operators to use in the pipeline stages, see Aggregation Pipeline Operators.

`db.aggregate()` Stages¶

Starting in version 3.6, MongoDB also provides the db.aggregate method:

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db.aggregate( [ { <stage> }, ... ] )

The following stages use the db.aggregate() method and not the db.collection.aggregate() method.

Stage	Description
`$currentOp`	Returns information on active and/or dormant operations for the MongoDB deployment.
`$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.

Stages Available for Updates¶

Starting in MongoDB 4.2, you can use the aggregation pipeline for updates in:

Command	`mongo` Shell Methods
`findAndModify`	db.collection.findOneAndUpdate() db.collection.findAndModify()
`update`	db.collection.updateOne() db.collection.updateMany() db.collection.update() Bulk.find.update() Bulk.find.updateOne() Bulk.find.upsert()

For the updates, the pipeline can consist of the following stages:

$addFields and its alias $set
$project and its alias $unset
$replaceRoot and its alias $replaceWith.

Alphabetical Listing of Stages¶

Name	Description
`$addFields`	Adds new fields to documents. Outputs documents that contain all existing fields from the input documents and newly added fields.
`$bucket`	Categorizes incoming documents into groups, called buckets, based on a specified expression and bucket boundaries.
`$bucketAuto`	Categorizes incoming documents into a specific number of groups, called buckets, based on a specified expression. Bucket boundaries are automatically determined in an attempt to evenly distribute the documents into the specified number of buckets.
`$collStats`	Returns statistics regarding a collection or view.
`$count`	Returns a count of the number of documents at this stage of the aggregation pipeline.
`$currentOp`	Returns information on active and/or dormant operations for the MongoDB deployment. To run, use the `db.aggregate()` method.
`$facet`	Processes multiple aggregation pipelines within a single stage on the same set of input documents. Enables the creation of multi-faceted aggregations capable of characterizing data across multiple dimensions, or facets, in a single stage.
`$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.
`$group`	Groups input documents by a specified identifier expression and applies the accumulator expression(s), if specified, to each group. Consumes all input documents and outputs one document per each distinct group. The output documents only contain the identifier field and, if specified, accumulated fields.
`$indexStats`	Returns statistics regarding the use of each index for the collection.
`$limit`	Passes the first n documents unmodified to the pipeline where n is the specified limit. For each input document, outputs either one document (for the first n documents) or zero documents (after the first n documents).
`$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.
`$listSessions`	Lists all sessions that have been active long enough to propagate to the `system.sessions` collection.
`$lookup`	Performs a left outer join to another collection in the same database to filter in documents from the “joined” collection 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. New in version 4.2.
`$out`	Writes the resulting documents of the aggregation pipeline to a collection. To use the `$out` stage, it must be the last stage in the pipeline.
`$planCacheStats`	Returns plan cache information for a collection.
`$project`	Reshapes each document in the stream, such as by adding new fields or removing existing fields. For each input document, outputs one document.
`$redact`	Reshapes each document in the stream by restricting the content for each document based on information stored in the documents themselves. Incorporates the functionality of `$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.
`$replaceWith`	Replaces a document with the specified embedded document. The operation replaces all existing fields in the input document, including the `_id` field. Specify a document embedded in the input document to promote the embedded document to the top level. Alias for `$replaceRoot`.
`$sample`	Randomly selects the specified number of documents from its input.
`$set`	Adds new fields to documents. Outputs documents that contain all existing fields from the input documents and newly added fields. Alias for `$addFields`.
`$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.
`$unionWith`	Performs a union of two collections; i.e. combines pipeline results from two collections into a single result set. New in version 4.4.
`$unset`	Removes/exludes fields from documents. Alias for `$project` stage that excludes/removes fields.
`$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.

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