db.collection.aggregate()¶

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Definition
Cursor Behavior
Examples

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.

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.

Changed in version 2.4: If an error occurs, the aggregate() helper throws an exception. In previous versions, the helper returned a document with the error message and code, and ok status field not equal to 1, same as the aggregate command.

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 Cursors for cursor behavior in the mongo shell and 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:

Examples¶

The examples in this section use the db.collection.aggregate() helper provided in the 2.6 version of the mongo shell.

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.

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

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