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db.aggregate()

Definition

New in version 3.6.

db.aggregate()

Runs a specified admin/diagnostic pipeline which does not require an underlying collection. For aggregations on collection data, see db.collection.aggregate().

The db.aggregate() method has the following syntax:

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

• The pipeline parameter is an array of stages to execute. It must start with a compatible stage that does not require an underlying collection, such as $currentOp or $listLocalSessions.

• 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. Not available in multi-document transactions.
  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. Starting in MongoDB 4.2, the profiler log messages and diagnostic log messages includes a usedDisk indicator if any aggregation stage wrote data to temporary files due to memory restrictions.
  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.
  maxTimeMS	non-negative integer	Optional. Specifies a time limit in milliseconds for processing operations on a cursor. If you do not specify a value for maxTimeMS, operations will not time out. A value of 0 explicitly specifies the default unbounded behavior. MongoDB terminates operations that exceed their allotted time limit using the same mechanism as db.killOp(). MongoDB only terminates an operation at one of its designated interrupt points.
  bypassDocumentValidation	boolean	Optional. Applicable only if you specify the $out or $merge aggregation stages. 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. Starting in MongoDB 3.6, the readConcern option has the following syntax: readConcern: { level: <value> } Possible read concern levels are: "local". This is the default read concern level for read operations against primary and read operations against secondaries when associated with causally consistent sessions. "available". This is the default for reads against secondaries when when not associated with causally consistent sessions. The query returns the instance’s most recent data. "majority". Available for replica sets that use WiredTiger storage engine. "linearizable". Available for read operations on the primary only. For more formation on the read concern levels, see Read Concern Levels. Starting in MongoDB 4.2, the $out stage cannot be used in conjunction with read concern "linearizable". That is, if you specify "linearizable" read concern for db.collection.aggregate(), you cannot include the $out stage in the pipeline. The $merge stage cannot be used in conjunction with read concern "linearizable". That is, if you specify "linearizable" read concern for db.collection.aggregate(), you cannot include the $merge stage in the pipeline.
  collation	document	Optional. Specifies the collation to use for the operation. Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks. The collation option has the following syntax: copy collation: { locale: <string>, caseLevel: <boolean>, caseFirst: <string>, strength: <int>, numericOrdering: <boolean>, alternate: <string>, maxVariable: <string>, backwards: <boolean> } When specifying collation, the locale field is mandatory; all other collation fields are optional. For descriptions of the fields, see Collation Document. If the collation is unspecified but the collection has a default collation (see db.createCollection()), the operation uses the collation specified for the collection. If no collation is specified for the collection or for the operations, MongoDB uses the simple binary comparison used in prior versions for string comparisons. You cannot specify multiple collations for an operation. For example, you cannot specify different collations per field, or if performing a find with a sort, you cannot use one collation for the find and another for the sort. New in version 3.4.
  hint	string or document	Optional. The index to use for the aggregation. The index is on the initial collection/view against which the aggregation is run. Specify the index either by the index name or by the index specification document. Note The hint does not apply to $lookup and $graphLookup stages. New in version 3.6.
  comment	string	Optional. Users can specify an arbitrary string to help trace the operation through the database profiler, currentOp, and logs. New in version 3.6.
  writeConcern	document	Optional. A document that expresses the write concern to use with the $out or $merge stage. Omit to use the default write concern with the $out or $merge stage.

Example

Pipeline with $currentOp

The following example runs a pipeline with two stages. The first stage runs the $currentOp operation and the second stage filters the results of that operation.

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use admin
db.aggregate( [ {
   $currentOp : { allUsers: true, idleConnections: true } }, {
   $match : { shard: "shard01" }
   }
] )

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