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db.collection.aggregate()

Definition

db.collection.aggregate(pipeline, options)

Calculates aggregate values for the data in a collection.

Parameter	Type	Description
pipeline	array	A sequence of data aggregation operations or stages. See the aggregation pipeline operators for details. Changed in version 2.6: The method can still accept the pipeline stages as separate arguments instead of as elements in an array; however, if you do not specify the pipeline as an array, you cannot specify the options parameter.
options	document	Optional. Additional options that aggregate() passes to the aggregate command. New in version 2.6: Available only if you specify the pipeline as an array.

The options document can contain the following fields and values:

Field	Type	Description
explain	boolean	Optional. Specifies to return the information on the processing of the pipeline. See Return Information on Aggregation Pipeline Operation for an example. New in version 2.6.
allowDiskUse	boolean	Optional. Enables writing to temporary files. When set to true, aggregation operations can write data to the _tmp subdirectory in the dbPath directory. See Perform Large Sort Operation with External Sort for an example. New in version 2.6.
cursor	document	Optional. Specifies the initial batch size for the cursor. The value of the cursor field is a document with the field batchSize. See Specify an Initial Batch Size for syntax and example. New in version 2.6.
maxTimeMS	non-negative integer	Optional. The maximum time period in milliseconds the getMore() operation will block waiting for new data to be inserted into the capped collection. Requires that the cursor on which this getMore() is acting is an awaitData cursor. See the awaitData parameter for find().
bypassDocumentValidation	boolean	Optional. Available only if you specify the $out aggregation operator. Enables db.collection.aggregate to bypass document validation during the operation. This lets you insert documents that do not meet the validation requirements. New in version 3.2.
readConcern	document	Optional. Specifies the read concern. The default level is "local". To use a read concern level of "majority", you must use the WiredTiger storage engine and start the mongod instances with the --enableMajorityReadConcern command line option (or the replication.enableMajorityReadConcern setting if using a configuration file). Only replica sets using protocol version 1 support "majority" read concern. Replica sets running protocol version 0 do not support "majority" read concern. To ensure that a single thread can read its own writes, use "majority" read concern and "majority" write concern against the primary of the replica set. To use a read concern level of "majority", you cannot include the $out stage. New in version 3.2.

Returns:A cursor to the documents produced by the final stage of the aggregation pipeline operation, or if you include the explain option, the document that provides details on the processing of the aggregation operation.

If the pipeline includes the $out operator, aggregate() returns an empty cursor. See $out for more information.

Changed in version 2.6: The db.collection.aggregate() method returns a cursor and can return result sets of any size. Previous versions returned all results in a single document, and the result set was subject to a size limit of 16 megabytes.

Behavior

Error Handling

If an error occurs, the aggregate() helper throws an exception.

Cursor Behavior

In the mongo shell, if the cursor returned from the db.collection.aggregate() is not assigned to a variable using the var keyword, then the mongo shell automatically iterates the cursor up to 20 times. See Iterate a Cursor in the mongo Shell for handling cursors in the mongo shell.

Cursors returned from aggregation only supports cursor methods that operate on evaluated cursors (i.e. cursors whose first batch has been retrieved), such as the following methods:

cursor.hasNext() cursor.next() cursor.toArray() cursor.forEach()

cursor.map() cursor.objsLeftInBatch() cursor.itcount() cursor.pretty()

See also

For more information, see Aggregation Pipeline, Aggregation Reference, Aggregation Pipeline Limits, and aggregate.

Examples

The following examples use the collection orders that contains the following documents:

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{ _id: 1, cust_id: "abc1", ord_date: ISODate("2012-11-02T17:04:11.102Z"), status: "A", amount: 50 }
{ _id: 2, cust_id: "xyz1", ord_date: ISODate("2013-10-01T17:04:11.102Z"), status: "A", amount: 100 }
{ _id: 3, cust_id: "xyz1", ord_date: ISODate("2013-10-12T17:04:11.102Z"), status: "D", amount: 25 }
{ _id: 4, cust_id: "xyz1", ord_date: ISODate("2013-10-11T17:04:11.102Z"), status: "D", amount: 125 }
{ _id: 5, cust_id: "abc1", ord_date: ISODate("2013-11-12T17:04:11.102Z"), status: "A", amount: 25 }

Group by and Calculate a Sum

The following aggregation operation selects documents with status equal to "A", groups the matching documents by the cust_id field and calculates the total for each cust_id field from the sum of the amount field, and sorts the results by the total field in descending order:

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db.orders.aggregate([
                     { $match: { status: "A" } },
                     { $group: { _id: "$cust_id", total: { $sum: "$amount" } } },
                     { $sort: { total: -1 } }
                   ])

The operation returns a cursor with the following documents:

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{ "_id" : "xyz1", "total" : 100 }
{ "_id" : "abc1", "total" : 75 }

The mongo shell iterates the returned cursor automatically to print the results. See Iterate a Cursor in the mongo Shell for handling cursors manually in the mongo shell.

Return Information on Aggregation Pipeline Operation

The following aggregation operation sets the option explain to true to return information about the aggregation operation.

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db.orders.aggregate(
                     [
                       { $match: { status: "A" } },
                       { $group: { _id: "$cust_id", total: { $sum: "$amount" } } },
                       { $sort: { total: -1 } }
                     ],
                     {
                       explain: true
                     }
                   )

The operation returns a cursor with the document that contains detailed information regarding the processing of the aggregation pipeline. For example, the document may show, among other details, which index, if any, the operation used. [1] If the orders collection is a sharded collection, the document would also show the division of labor between the shards and the merge operation, and for targeted queries, the targeted shards.

Note

The intended readers of the explain output document are humans, and not machines, and the output format is subject to change between releases.

The mongo shell iterates the returned cursor automatically to print the results. See Iterate a Cursor in the mongo Shell for handling cursors manually in the mongo shell.

Perform Large Sort Operation with External Sort

Aggregation pipeline stages have maximum memory use limit. To handle large datasets, set allowDiskUse option to true to enable writing data to temporary files, as in the following example:

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var results = db.stocks.aggregate(
                                   [
                                     { $project : { cusip: 1, date: 1, price: 1, _id: 0 } },
                                     { $sort : { cusip : 1, date: 1 } }
                                   ],
                                   {
                                     allowDiskUse: true
                                   }
                                 )

Specify an Initial Batch Size

To specify an initial batch size for the cursor, use the following syntax for the cursor option:

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cursor: { batchSize: <int> }

For example, the following aggregation operation specifies the initial batch size of 0 for the cursor:

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db.orders.aggregate(
                     [
                       { $match: { status: "A" } },
                       { $group: { _id: "$cust_id", total: { $sum: "$amount" } } },
                       { $sort: { total: -1 } },
                       { $limit: 2 }
                     ],
                     {
                       cursor: { batchSize: 0 }
                     }
                   )

A batchSize of 0 means an empty first batch and is useful for quickly returning a cursor or failure message without doing significant server-side work. Specify subsequent batch sizes to OP_GET_MORE operations as with other MongoDB cursors.

The mongo shell iterates the returned cursor automatically to print the results. See Iterate a Cursor in the mongo Shell for handling cursors manually in the mongo shell.

Override readConcern

The following operation on a replica set specifies a Read Concern of "majority" to read the most recent copy of the data confirmed as having been written to a majority of the nodes.

Note

• To use a read concern level of "majority", you must use the WiredTiger storage engine and start the mongod instances with the --enableMajorityReadConcern command line option (or the replication.enableMajorityReadConcern setting if using a configuration file).

  Only replica sets using protocol version 1 support "majority" read concern. Replica sets running protocol version 0 do not support "majority" read concern.

• To ensure that a single thread can read its own writes, use "majority" read concern and "majority" write concern against the primary of the replica set.

• To use a read concern level of "majority", you cannot include the $out stage.

• Regardless of the read concern level, the most recent data on a node may not reflect the most recent version of the data in the system.

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db.restaurants.aggregate(
   [ { $match: { rating: { $lt: 5 } } } ],
   { readConcern: { level: "majority" } }
)

[1]	Index Filters can affect the choice of index used. See Index Filters for details.

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