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- db.collection.aggregate()
db.collection.aggregate()¶
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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 theoptions
parameter.options
document Optional. Additional options that
aggregate()
passes to theaggregate
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 thedbPath
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 fieldbatchSize
. 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 anawaitData
cursor. See theawaitData
parameter forfind()
.bypassDocumentValidation
boolean Optional. Available only if you specify the
$out
aggregation operator.Enables
db.collection.aggregate
to bypass document validation during the operation. This lets you insert documents that do not meet the validation requirements.New in version 3.2.
readConcern
document Optional. Specifies the read concern. The default level is
"local"
.To use a read concern level of
"majority"
, you must use the WiredTiger storage engine and start themongod
instances with the--enableMajorityReadConcern
command line option (or thereplication.enableMajorityReadConcern
setting if using a configuration file).Only replica sets using
protocol version 1
support"majority"
read concern. Replica sets running protocol version 0 do not support"majority"
read concern.To ensure that a single thread can read its own writes, use
"majority"
read concern and"majority"
write concern against the primary of the replica set.To use a read concern level of
"majority"
, you cannot include the$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:
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:
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:
The operation returns a cursor with the following documents:
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.
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:
Specify an Initial Batch Size¶
To specify an initial batch size for the cursor, use the following
syntax for the cursor
option:
For example, the following aggregation operation specifies the
initial batch size of 0
for the cursor:
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 themongod
instances with the--enableMajorityReadConcern
command line option (or thereplication.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.
[1] | Index Filters can affect the choice of index used. See Index Filters for details. |