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$out (aggregation)

New in version 2.6.



Takes the documents returned by the aggregation pipeline and writes them to a specified collection. The $out operator must be the last stage in the pipeline. The $out operator lets the aggregation framework return result sets of any size.

Changed in version 3.2.0: MongoDB 3.2 added support for document validation. The bypassDocumentValidation field enables you to bypass document validation during the $out phase of the aggregation operation. This lets you insert documents that do not meet the validation requirements. Specify bypassDocumentValidation as an option on the aggregation method or command.

The $out stage has the following prototype form:

{ $out: "<output-collection>" }

$out takes a string that specifies the output collection name.


  • You cannot specify a sharded collection as the output collection. The input collection for a pipeline can be sharded.
  • The $out operator cannot write results to a capped collection.


Create New Collection

The $out operation creates a new collection in the current database if one does not already exist. The collection is not visible until the aggregation completes. If the aggregation fails, MongoDB does not create the collection.

Replace Existing Collection

If the collection specified by the $out operation already exists, then upon completion of the aggregation, the $out stage atomically replaces the existing collection with the new results collection. The $out operation does not change any indexes that existed on the previous collection. If the aggregation fails, the $out operation makes no changes to the pre-existing collection.

Index Constraints

The pipeline will fail to complete if the documents produced by the pipeline would violate any unique indexes, including the index on the _id field of the original output collection.


A collection books contains the following documents:

{ "_id" : 8751, "title" : "The Banquet", "author" : "Dante", "copies" : 2 }
{ "_id" : 8752, "title" : "Divine Comedy", "author" : "Dante", "copies" : 1 }
{ "_id" : 8645, "title" : "Eclogues", "author" : "Dante", "copies" : 2 }
{ "_id" : 7000, "title" : "The Odyssey", "author" : "Homer", "copies" : 10 }
{ "_id" : 7020, "title" : "Iliad", "author" : "Homer", "copies" : 10 }

The following aggregation operation pivots the data in the books collection to have titles grouped by authors and then writes the results to the authors collection.

db.books.aggregate( [
                      { $group : { _id : "$author", books: { $push: "$title" } } },
                      { $out : "authors" }
                  ] )

After the operation, the authors collection contains the following documents:

{ "_id" : "Homer", "books" : [ "The Odyssey", "Iliad" ] }
{ "_id" : "Dante", "books" : [ "The Banquet", "Divine Comedy", "Eclogues" ] }