Mongo db aggregation-guide

MongoDB Aggregation and Data
Processing
Release 2.6.4
MongoDB Documentation Project
September 16, 2014
Contents
1 Aggregation Introduction 3
1.1 Aggregation Modalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Aggregation Pipelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Map-Reduce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Single Purpose Aggregation Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2 Additional Features and Behaviors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Aggregation Concepts 7
2.1 Aggregation Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Pipeline Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Aggregation Pipeline Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Map-Reduce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Map-Reduce JavaScript Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Map-Reduce Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3 Single Purpose Aggregation Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Count . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Distinct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4 Aggregation Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Aggregation Pipeline Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Aggregation Pipeline Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Aggregation Pipeline and Sharded Collections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Map-Reduce and Sharded Collections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Map Reduce Concurrency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3 Aggregation Examples 19
3.1 Aggregation with the Zip Code Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Data Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Return States with Populations above 10 Million . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Return Average City Population by State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Return Largest and Smallest Cities by State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2 Aggregation with User Preference Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Data Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Normalize and Sort Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Return Usernames Ordered by Join Month . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Return Total Number of Joins per Month . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Return the Five Most Common “Likes” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.3 Map-Reduce Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Return the Total Price Per Customer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Calculate Order and Total Quantity with Average Quantity Per Item . . . . . . . . . . . . . . . . . . 28
3.4 Perform Incremental Map-Reduce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Data Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Initial Map-Reduce of Current Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Subsequent Incremental Map-Reduce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5 Troubleshoot the Map Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.6 Troubleshoot the Reduce Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Confirm Output Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Ensure Insensitivity to the Order of Mapped Values . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Ensure Reduce Function Idempotence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4 Aggregation Reference 35
4.1 Aggregation Pipeline Quick Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Accumulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.2 Aggregation Commands Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.3 SQL to Aggregation Mapping Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.4 Aggregation Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Aggregation Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Aggregation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.5 Variables in Aggregation Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
User Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
System Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Index 46
Aggregations operations process data records and return computed results. Aggregation operations group values from
multiple documents together, and can perform a variety of operations on the grouped data to return a single result.
MongoDB provides three ways to perform aggregation: the aggregation pipeline (page 7), the map-reduce function
(page 10), and single purpose aggregation methods and commands (page 11).
Aggregation Introduction (page 3) A high-level introduction to aggregation.
Aggregation Concepts (page 7) Introduces the use and operation of the data aggregation modalities available in Mon-goDB.
Aggregation Pipeline (page 7) The aggregation pipeline is a framework for performing aggregation tasks,
modeled on the concept of data processing pipelines. Using this framework, MongoDB passes the doc-uments
of a single collection through a pipeline. The pipeline transforms the documents into aggregated
results, and is accessed through the aggregate database command.
Map-Reduce (page 10) Map-reduce is a generic multi-phase data aggregation modality for processing quanti-ties
of data. MongoDB provides map-reduce with the mapReduce database command.
Single Purpose Aggregation Operations (page 11) MongoDB provides a collection of specific data aggrega-tion
operations to support a number of common data aggregation functions. These operations include
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returning counts of documents, distinct values of a field, and simple grouping operations.
Aggregation Mechanics (page 14) Details internal optimization operations, limits, support for sharded collec-tions,
and concurrency concerns.
Aggregation Examples (page 19) Examples and tutorials for data aggregation operations in MongoDB.
Aggregation Reference (page 35) References for all aggregation operations material for all data aggregation methods
in MongoDB.
1 Aggregation Introduction
Aggregations are operations that process data records and return computed results. MongoDB provides a rich set
of aggregation operations that examine and perform calculations on the data sets. Running data aggregation on the
mongod instance simplifies application code and limits resource requirements.
Like queries, aggregation operations in MongoDB use collections of documents as an input and return results in the
form of one or more documents.
1.1 Aggregation Modalities
Aggregation Pipelines
MongoDB 2.2 introduced a new aggregation framework (page 7), modeled on the concept of data processing pipelines.
Documents enter a multi-stage pipeline that transforms the documents into an aggregated result.
The most basic pipeline stages provide filters that operate like queries and document transformations that modify the
form of the output document.
Other pipeline operations provide tools for grouping and sorting documents by specific field or fields as well as tools
for aggregating the contents of arrays, including arrays of documents. In addition, pipeline stages can use operators
for tasks such as calculating the average or concatenating a string.
The pipeline provides efficient data aggregation using native operations within MongoDB, and is the preferred method
for data aggregation in MongoDB.
Map-Reduce
MongoDB also provides map-reduce (page 10) operations to perform aggregation. In general, map-reduce operations
have two phases: a map stage that processes each document and emits one or more objects for each input document,
and reduce phase that combines the output of the map operation. Optionally, map-reduce can have a finalize stage to
make final modifications to the result. Like other aggregation operations, map-reduce can specify a query condition to
select the input documents as well as sort and limit the results.
Map-reduce uses custom JavaScript functions to perform the map and reduce operations, as well as the optional finalize
operation. While the custom JavaScript provide great flexibility compared to the aggregation pipeline, in general, map-reduce
is less efficient and more complex than the aggregation pipeline.
Note: Starting in MongoDB 2.4, certain mongo shell functions and properties are inaccessible in map-reduce op-erations.
MongoDB 2.4 also provides support for multiple JavaScript operations to run at the same time. Before
MongoDB 2.4, JavaScript code executed in a single thread, raising concurrency issues for map-reduce.
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Figure 1: Diagram of the annotated aggregation pipeline operation. The aggregation pipeline has two stages: $match
and $group.
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Figure 2: Diagram of the annotated map-reduce operation.
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Single Purpose Aggregation Operations
For a number of common single purpose aggregation operations (page 11), MongoDB provides special purpose
database commands. These common aggregation operations are: returning a count of matching documents, returning
the distinct values for a field, and grouping data based on the values of a field. All of these operations aggregate
documents from a single collection. While these operations provide simple access to common aggregation processes,
they lack the flexibility and capabilities of the aggregation pipeline and map-reduce.
Figure 3: Diagram of the annotated distinct operation.
1.2 Additional Features and Behaviors
Both the aggregation pipeline and map-reduce can operate on a sharded collection. Map-reduce operations
can also output to a sharded collection. See Aggregation Pipeline and Sharded Collections (page 17) and Map-Reduce
and Sharded Collections (page 18) for details.
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The aggregation pipeline can use indexes to improve its performance during some of its stages. In addition, the
aggregation pipeline has an internal optimization phase. See Pipeline Operators and Indexes (page 9) and Aggregation
Pipeline Optimization (page 14) for details.
For a feature comparison of the aggregation pipeline, map-reduce, and the special group functionality, see Aggregation
Commands Comparison (page 40).
2 Aggregation Concepts
MongoDB provides the three approaches to aggregation, each with its own strengths and purposes for a given situation.
This section describes these approaches and also describes behaviors and limitations specific to each approach. See
also the chart (page 40) that compares the approaches.
Aggregation Pipeline (page 7) The aggregation pipeline is a framework for performing aggregation tasks, modeled
on the concept of data processing pipelines. Using this framework, MongoDB passes the documents of a single
collection through a pipeline. The pipeline transforms the documents into aggregated results, and is accessed
through the aggregate database command.
Map-Reduce (page 10) Map-reduce is a generic multi-phase data aggregation modality for processing quantities of
data. MongoDB provides map-reduce with the mapReduce database command.
Single Purpose Aggregation Operations (page 11) MongoDB provides a collection of specific data aggregation op-erations
to support a number of common data aggregation functions. These operations include returning counts
of documents, distinct values of a field, and simple grouping operations.
Aggregation Mechanics (page 14) Details internal optimization operations, limits, support for sharded collections,
and concurrency concerns.
2.1 Aggregation Pipeline
New in version 2.2.
The aggregation pipeline is a framework for data aggregation modeled on the concept of data processing pipelines.
Documents enter a multi-stage pipeline that transforms the documents into an aggregated results.
The aggregation pipeline provides an alternative to map-reduce and may be the preferred solution for aggregation tasks
where the complexity of map-reduce may be unwarranted.
Aggregation pipeline have some limitations on value types and result size. See Aggregation Pipeline Limits (page 17)
for details on limits and restrictions on the aggregation pipeline.
Pipeline
The MongoDB aggregation pipeline consists of stages. Each stage transforms the documents as they pass through the
pipeline. Pipeline stages do not need to produce one output document for every input document; e.g., some stages may
generate new documents or filter out documents. Pipeline stages can appear multiple times in the pipeline.
MongoDB provides the db.collection.aggregate() method in the mongo shell and the aggregate com-mand
for aggregation pipeline. See aggregation-pipeline-operator-reference for the available stages.
For example usage of the aggregation pipeline, consider Aggregation with User Preference Data (page 23) and Aggre-gation
with the Zip Code Data Set (page 20).
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Figure 4: Diagram of the annotated aggregation pipeline operation. The aggregation pipeline has two stages: $match
and $group.
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Pipeline Expressions
Some pipeline stages takes a pipeline expression as its operand. Pipeline expressions specify the transformation to
apply to the input documents. Expressions have a document structure and can contain other expression (page 36).
Pipeline expressions can only operate on the current document in the pipeline and cannot refer to data from other
documents: expression operations provide in-memory transformation of documents.
Generally, expressions are stateless and are only evaluated when seen by the aggregation process with one exception:
accumulator expressions.
The accumulators, used with the $group pipeline operator, maintain their state (e.g. totals, maximums, minimums,
and related data) as documents progress through the pipeline.
For more information on expressions, see Expressions (page 36).
Aggregation Pipeline Behavior
In MongoDB, the aggregate command operates on a single collection, logically passing the entire collection into
the aggregation pipeline. To optimize the operation, wherever possible, use the following strategies to avoid scanning
the entire collection.
Pipeline Operators and Indexes
The $match and $sort pipeline operators can take advantage of an index when they occur at the beginning of the
pipeline.
New in version 2.4: The $geoNear pipeline operator takes advantage of a geospatial index. When using $geoNear,
the $geoNear pipeline operation must appear as the first stage in an aggregation pipeline.
Even when the pipeline uses an index, aggregation still requires access to the actual documents; i.e. indexes cannot
fully cover an aggregation pipeline.
Changed in version 2.6: In previous versions, for very select use cases, an index could cover a pipeline.
Early Filtering
If your aggregation operation requires only a subset of the data in a collection, use the $match, $limit, and $skip
stages to restrict the documents that enter at the beginning of the pipeline. When placed at the beginning of a pipeline,
$match operations use suitable indexes to scan only the matching documents in a collection.
Placing a $match pipeline stage followed by a $sort stage at the start of the pipeline is logically equivalent to a
single query with a sort and can use an index. When possible, place $match operators at the beginning of the pipeline.
Additional Features
The aggregation pipeline has an internal optimization phase that provides improved performance for certain sequences
of operators. For details, see Aggregation Pipeline Optimization (page 14).
The aggregation pipeline supports operations on sharded collections. See Aggregation Pipeline and Sharded Collec-tions
(page 17).
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2.2 Map-Reduce
Map-reduce is a data processing paradigm for condensing large volumes of data into useful aggregated results. For
map-reduce operations, MongoDB provides the mapReduce database command.
Consider the following map-reduce operation:
Figure 5: Diagram of the annotated map-reduce operation.
In this map-reduce operation, MongoDB applies the map phase to each input document (i.e. the documents in the
collection that match the query condition). The map function emits key-value pairs. For those keys that have multiple
values, MongoDB applies the reduce phase, which collects and condenses the aggregated data. MongoDB then stores
the results in a collection. Optionally, the output of the reduce function may pass through a finalize function to further
condense or process the results of the aggregation.
All map-reduce functions in MongoDB are JavaScript and run within the mongod process. Map-reduce operations
take the documents of a single collection as the input and can perform any arbitrary sorting and limiting before
beginning the map stage. mapReduce can return the results of a map-reduce operation as a document, or may write
the results to collections. The input and the output collections may be sharded.
Note: For most aggregation operations, the Aggregation Pipeline (page 7) provides better performance and more
coherent interface. However, map-reduce operations provide some flexibility that is not presently available in the
aggregation pipeline.
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Map-Reduce JavaScript Functions
In MongoDB, map-reduce operations use custom JavaScript functions to map, or associate, values to a key. If a key
has multiple values mapped to it, the operation reduces the values for the key to a single object.
The use of custom JavaScript functions provide flexibility to map-reduce operations. For instance, when processing a
document, the map function can create more than one key and value mapping or no mapping. Map-reduce operations
can also use a custom JavaScript function to make final modifications to the results at the end of the map and reduce
operation, such as perform additional calculations.
Map-Reduce Behavior
In MongoDB, the map-reduce operation can write results to a collection or return the results inline. If you write
map-reduce output to a collection, you can perform subsequent map-reduce operations on the same input collection
that merge replace, merge, or reduce new results with previous results. See mapReduce and Perform Incremental
Map-Reduce (page 29) for details and examples.
When returning the results of a map reduce operation inline, the result documents must be within the BSON
Document Size limit, which is currently 16 megabytes. For additional information on limits and restrictions on
map-reduce operations, see the http://docs.mongodb.org/manualreference/command/mapReduce
reference page.
MongoDB supports map-reduce operations on sharded collections. Map-reduce operations can also output
the results to a sharded collection. See Map-Reduce and Sharded Collections (page 18).
2.3 Single Purpose Aggregation Operations
Aggregation refers to a broad class of data manipulation operations that compute a result based on an input and a spe-cific
procedure. MongoDB provides a number of aggregation operations that perform specific aggregation operations
on a set of data.
Although limited in scope, particularly compared to the aggregation pipeline (page 7) and map-reduce (page 10), these
operations provide straightforward semantics for common data processing options.
Count
MongoDB can return a count of the number of documents that match a query. The count command as well as the
count() and cursor.count() methods provide access to counts in the mongo shell.
Example
Given a collection named records with only the following documents:
{ a: 1, b: 0 }
{ a: 1, b: 1 }
{ a: 1, b: 4 }
{ a: 2, b: 2 }
The following operation would count all documents in the collection and return the number 4:
db.records.count()
The following operation will count only the documents where the value of the field a is 1 and return 3:
db.records.count( { a: 1 } )
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Distinct
The distinct operation takes a number of documents that match a query and returns all of the unique values for a field
in the matching documents. The distinct command and db.collection.distinct() method provide this
operation in the mongo shell. Consider the following examples of a distinct operation:
Figure 6: Diagram of the annotated distinct operation.
Example
Given a collection named records with only the following documents:
{ a: 1, b: 0 }
{ a: 1, b: 1 }
{ a: 1, b: 1 }
{ a: 1, b: 4 }
{ a: 2, b: 2 }
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{ a: 2, b: 2 }
Consider the following db.collection.distinct() operation which returns the distinct values of the field b:
db.records.distinct( "b" )
The results of this operation would resemble:
[ 0, 1, 4, 2 ]
Group
The group operation takes a number of documents that match a query, and then collects groups of documents based
on the value of a field or fields. It returns an array of documents with computed results for each group of documents.
Access the grouping functionality via the group command or the db.collection.group() method in the
mongo shell.
Warning: group does not support data in sharded collections. In addition, the results of the group operation
must be no larger than 16 megabytes.
Consider the following group operation:
Example
Given a collection named records with the following documents:
{ a: 1, count: 4 }
{ a: 1, count: 2 }
{ a: 1, count: 4 }
{ a: 2, count: 3 }
{ a: 2, count: 1 }
{ a: 1, count: 5 }
{ a: 4, count: 4 }
Consider the following group operation which groups documents by the field a, where a is less than 3, and sums the
field count for each group:
db.records.group( {
key: { a: 1 },
cond: { a: { $lt: 3 } },
reduce: function(cur, result) { result.count += cur.count },
initial: { count: 0 }
} )
The results of this group operation would resemble the following:
[
{ a: 1, count: 15 },
{ a: 2, count: 4 }
]
See also:
The $group for related functionality in the aggregation pipeline (page 7).
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2.4 Aggregation Mechanics
This section describes behaviors and limitations for the various aggregation modalities.
Aggregation Pipeline Optimization (page 14) Details the internal optimization of certain pipeline sequence.
Aggregation Pipeline Limits (page 17) Presents limitations on aggregation pipeline operations.
Aggregation Pipeline and Sharded Collections (page 17) Mechanics of aggregation pipeline operations on sharded
collections.
Map-Reduce and Sharded Collections (page 18) Mechanics of map-reduce operation with sharded collections.
Map Reduce Concurrency (page 19) Details the locks taken during map-reduce operations.
Aggregation Pipeline Optimization
Aggregation pipeline operations have an optimization phase which attempts to reshape the pipeline for improved
performance.
To see how the optimizer transforms a particular aggregation pipeline, include the explain option in the
db.collection.aggregate() method.
Optimizations are subject to change between releases.
Projection Optimization
The aggregation pipeline can determine if it requires only a subset of the fields in the documents to obtain the results.
If so, the pipeline will only use those required fields, reducing the amount of data passing through the pipeline.
Pipeline Sequence Optimization
$sort + $match Sequence Optimization When you have a sequence with $sort followed by a $match, the
$match moves before the $sort to minimize the number of objects to sort. For example, if the pipeline consists of
the following stages:
{ $sort: { age : -1 } },
{ $match: { status: 'A' } }
During the optimization phase, the optimizer transforms the sequence to the following:
{ $match: { status: 'A' } },
{ $sort: { age : -1 } }
$skip + $limit Sequence Optimization When you have a sequence with $skip followed by a $limit, the
$limit moves before the $skip. With the reordering, the $limit value increases by the $skip amount.
For example, if the pipeline consists of the following stages:
{ $skip: 10 },
{ $limit: 5 }
During the optimization phase, the optimizer transforms the sequence to the following:
{ $limit: 15 },
{ $skip: 10 }
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This optimization allows for more opportunities for $sort + $limit Coalescence (page 15), such as with $sort +
$skip + $limit sequences. See $sort + $limit Coalescence (page 15) for details on the coalescence and $sort +
$skip + $limit Sequence (page 16) for an example.
For aggregation operations on sharded collections (page 17), this optimization reduces the results returned from each
shard.
$redact + $match Sequence Optimization When possible, when the pipeline has the $redact stage immedi-ately
followed by the $match stage, the aggregation can sometimes add a portion of the $match stage before the
$redact stage. If the added $match stage is at the start of a pipeline, the aggregation can use an index as well
as query the collection to limit the number of documents that enter the pipeline. See Pipeline Operators and Indexes
(page 9) for more information.
For example, if the pipeline consists of the following stages:
{ $redact: { $cond: { if: { $eq: [ "$level", 5 ] }, then: "$$PRUNE", else: "$$DESCEND" } } },
{ $match: { year: 2014, category: { $ne: "Z" } } }
The optimizer can add the same $match stage before the $redact stage:
{ $match: { year: 2014 } },
{ $redact: { $cond: { if: { $eq: [ "$level", 5 ] }, then: "$$PRUNE", else: "$$DESCEND" } } },
{ $match: { year: 2014, category: { $ne: "Z" } } }
Pipeline Coalescence Optimization
When possible, the optimization phase coalesces a pipeline stage into its predecessor. Generally, coalescence occurs
after any sequence reordering optimization.
$sort + $limit Coalescence When a $sort immediately precedes a $limit, the optimizer can coalesce the
$limit into the $sort. This allows the sort operation to only maintain the top n results as it progresses, where
n is the specified limit, and MongoDB only needs to store n items in memory 1. See sort-and-memory for more
information.
$limit + $limit Coalescence When a $limit immediately follows another $limit, the two stages can
coalesce into a single $limit where the limit amount is the smaller of the two initial limit amounts. For example, a
pipeline contains the following sequence:
{ $limit: 100 },
{ $limit: 10 }
Then the second $limit stage can coalesce into the first $limit stage and result in a single $limit stage where
the limit amount 10 is the minimum of the two initial limits 100 and 10.
{ $limit: 10 }
$skip + $skip Coalescence When a $skip immediately follows another $skip, the two stages can coalesce
into a single $skip where the skip amount is the sum of the two initial skip amounts. For example, a pipeline contains
the following sequence:
{ $skip: 5 },
{ $skip: 2 }
1 The optimization will still apply when allowDiskUse is true and the n items exceed the aggregation memory limit (page 17).
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Then the second $skip stage can coalesce into the first $skip stage and result in a single $skip stage where the
skip amount 7 is the sum of the two initial limits 5 and 2.
{ $skip: 7 }
$match + $match Coalescence When a $match immediately follows another $match, the two stages can
coalesce into a single $match combining the conditions with an $and. For example, a pipeline contains the following
sequence:
{ $match: { year: 2014 } },
{ $match: { status: "A" } }
Then the second $match stage can coalesce into the first $match stage and result in a single $match stage
{ $match: { $and: [ { "year" : 2014 }, { "status" : "A" } ] } }
Examples
The following examples are some sequences that can take advantage of both sequence reordering and coalescence.
Generally, coalescence occurs after any sequence reordering optimization.
$sort + $skip + $limit Sequence A pipeline contains a sequence of $sort followed by a $skip followed
by a $limit:
{ $sort: { age : -1 } },
{ $skip: 10 },
{ $limit: 5 }
First, the optimizer performs the $skip + $limit Sequence Optimization (page 14) to transforms the sequence to the
following:
{ $sort: { age : -1 } },
{ $limit: 15 }
{ $skip: 10 }
The $skip + $limit Sequence Optimization (page 14) increases the $limit amount with the reordering. See $skip +
$limit Sequence Optimization (page 14) for details.
The reordered sequence now has $sort immediately preceding the $limit, and the pipeline can coalesce the two
stages to decrease memory usage during the sort operation. See $sort + $limit Coalescence (page 15) for more
information.
$limit + $skip + $limit + $skip Sequence A pipeline contains a sequence of alternating $limit and
$skip stages:
{ $limit: 100 },
{ $skip: 5 },
{ $limit: 10 },
{ $skip: 2 }
The $skip + $limit Sequence Optimization (page 14) reverses the position of the { $skip: 5 } and { $limit:
10 } stages and increases the limit amount:
16

{ $limit: 100 },
{ $limit: 15},
{ $skip: 5 },
{ $skip: 2 }
The optimizer then coalesces the two $limit stages into a single $limit stage and the two $skip stages into a
single $skip stage. The resulting sequence is the following:
{ $limit: 15 },
{ $skip: 7 }
See $limit + $limit Coalescence (page 15) and $skip + $skip Coalescence (page 15) for details.
See also:
explain option in the db.collection.aggregate()
Aggregation Pipeline Limits
Aggregation operations with the aggregate command have the following limitations.
Result Size Restrictions
If the aggregate command returns a single document that contains the complete result set, the command will
produce an error if the result set exceeds the BSON Document Size limit, which is currently 16 megabytes. To
manage result sets that exceed this limit, the aggregate command can return result sets of any size if the command
return a cursor or store the results to a collection.
Changed in version 2.6: The aggregate command can return results as a cursor or store the results in a collection,
which are not subject to the size limit. The db.collection.aggregate() returns a cursor and can return result
sets of any size.
Memory Restrictions
Changed in version 2.6.
Pipeline stages have a limit of 100 megabytes of RAM. If a stage exceeds this limit, MongoDB will produce an error.
To allow for the handling of large datasets, use the allowDiskUse option to enable aggregation pipeline stages to
write data to temporary files.
See also:
sort-memory-limit and group-memory-limit.
Aggregation Pipeline and Sharded Collections
The aggregation pipeline supports operations on sharded collections. This section describes behaviors specific to the
aggregation pipeline (page 7) and sharded collections.
Behavior
17

When operating on a sharded collection, the aggregation pipeline is split into two parts. The first pipeline runs on each
shard, or if an early $match can exclude shards through the use of the shard key in the predicate, the pipeline runs on
only the relevant shards.
The second pipeline consists of the remaining pipeline stages and runs on the primary shard. The primary shard
merges the cursors from the other shards and runs the second pipeline on these results. The primary shard forwards
the final results to the mongos. In previous versions, the second pipeline would run on the mongos. 2
Optimization
When splitting the aggregation pipeline into two parts, the pipeline is split to ensure that the shards perform as many
stages as possible with consideration for optimization.
To see how the pipeline was split, include the explain option in the db.collection.aggregate() method.
Optimizations are subject to change between releases.
Map-Reduce and Sharded Collections
Map-reduce supports operations on sharded collections, both as an input and as an output. This section describes the
behaviors of mapReduce specific to sharded collections.
Sharded Collection as Input
When using sharded collection as the input for a map-reduce operation, mongos will automatically dispatch the map-reduce
job to each shard in parallel. There is no special option required. mongos will wait for jobs on all shards to
finish.
Sharded Collection as Output
If the out field for mapReduce has the sharded value, MongoDB shards the output collection using the _id field
as the shard key.
To output to a sharded collection:
• If the output collection does not exist, MongoDB creates and shards the collection on the _id field.
• For a new or an empty sharded collection, MongoDB uses the results of the first stage of the map-reduce
operation to create the initial chunks distributed among the shards.
• mongos dispatches, in parallel, a map-reduce post-processing job to every shard that owns a chunk. During
the post-processing, each shard will pull the results for its own chunks from the other shards, run the final
reduce/finalize, and write locally to the output collection.
Note:
• During later map-reduce jobs, MongoDB splits chunks as needed.
• Balancing of chunks for the output collection is automatically prevented during post-processing to avoid con-currency
issues.
In MongoDB 2.0:
2 Until all shards upgrade to v2.6, the second pipeline runs on the mongos if any shards are still running v2.4.
18

• mongos retrieves the results from each shard, performs a merge sort to order the results, and proceeds to the
reduce/finalize phase as needed. mongos then writes the result to the output collection in sharded mode.
• This model requires only a small amount of memory, even for large data sets.
• Shard chunks are not automatically split during insertion. This requires manual intervention until the chunks
are granular and balanced.
Important: For best results, only use the sharded output options for mapReduce in version 2.2 or later.
Map Reduce Concurrency
The map-reduce operation is composed of many tasks, including reads from the input collection, executions of the
map function, executions of the reduce function, writes to a temporary collection during processing, and writes to
the output collection.
During the operation, map-reduce takes the following locks:
• The read phase takes a read lock. It yields every 100 documents.
• The insert into the temporary collection takes a write lock for a single write.
• If the output collection does not exist, the creation of the output collection takes a write lock.
• If the output collection exists, then the output actions (i.e. merge, replace, reduce) take a write lock. This
write lock is global, and blocks all operations on the mongod instance.
Changed in version 2.4: The V8 JavaScript engine, which became the default in 2.4, allows multiple JavaScript
operations to execute at the same time. Prior to 2.4, JavaScript code (i.e. map, reduce, finalize functions)
executed in a single thread.
Note: The final write lock during post-processing makes the results appear atomically. However, output actions
merge and reduce may take minutes to process. For the merge and reduce, the nonAtomic flag is available,
which releases the lock between writing each output document. the db.collection.mapReduce() reference
for more information.
3 Aggregation Examples
This document provides the practical examples that display the capabilities of aggregation (page 7).
Aggregation with the Zip Code Data Set (page 20) Use the aggregation pipeline to group values and to calculate ag-gregated
sums and averages for a collection of United States zip codes.
Aggregation with User Preference Data (page 23) Use the pipeline to sort, normalize, and sum data on a collection
of user data.
Map-Reduce Examples (page 27) Define map-reduce operations that select ranges, group data, and calculate sums
and averages.
Perform Incremental Map-Reduce (page 29) Run a map-reduce operations over one collection and output results to
another collection.
Troubleshoot the Map Function (page 31) Steps to troubleshoot the map function.
Troubleshoot the Reduce Function (page 32) Steps to troubleshoot the reduce function.
19

3.1 Aggregation with the Zip Code Data Set
The examples in this document use the zipcode collection. This collection is available at: me-dia.
mongodb.org/zips.json3. Use mongoimport to load this data set into your mongod instance.
Data Model
Each document in the zipcode collection has the following form:
{
"_id": "10280",
"city": "NEW YORK",
"state": "NY",
"pop": 5574,
"loc": [
-74.016323,
40.710537
]
}
The _id field holds the zip code as a string.
The city field holds the city name. A city can have more than one zip code associated with it as different sections of
the city can each have a different zip code.
The state field holds the two letter state abbreviation.
The pop field holds the population.
The loc field holds the location as a latitude longitude pair.
All of the following examples use the aggregate() helper in the mongo shell. aggregate() provides a wrapper
around the aggregate database command. See the documentation for your driver for a more idiomatic interface
for data aggregation operations.
Return States with Populations above 10 Million
To return all states with a population greater than 10 million, use the following aggregation operation:
db.zipcodes.aggregate( { $group :
{ _id : "$state",
totalPop : { $sum : "$pop" } } },
{ $match : {totalPop : { $gte : 10*1000*1000 } } } )
Aggregations operations using the aggregate() helper process all documents in the zipcodes collection.
aggregate() connects a number of pipeline (page 7) operators, which define the aggregation process.
In this example, the pipeline passes all documents in the zipcodes collection through the following steps:
• the $group operator collects all documents and creates documents for each state.
These new per-state documents have one field in addition to the _id field: totalPop which is a generated
field using the $sum operation to calculate the total value of all pop fields in the source documents.
After the $group operation the documents in the pipeline resemble the following:
3http://media.mongodb.org/zips.json
20

{
"_id" : "AK",
"totalPop" : 550043
}
• the $match operation filters these documents so that the only documents that remain are those where the value
of totalPop is greater than or equal to 10 million.
The $match operation does not alter the documents, which have the same format as the documents output by
$group.
The equivalent SQL for this operation is:
SELECT state, SUM(pop) AS totalPop
FROM zipcodes
GROUP BY state
HAVING totalPop >= (10*1000*1000)
Return Average City Population by State
To return the average populations for cities in each state, use the following aggregation operation:
db.zipcodes.aggregate( [
{ $group : { _id : { state : "$state", city : "$city" }, pop : { $sum : "$pop" } } },
{ $group : { _id : "$_id.state", avgCityPop : { $avg : "$pop" } } }
] )
Aggregations operations using the aggregate() helper process all documents in the zipcodes collection.
aggregate() connects a number of pipeline (page 7) operators that define the aggregation process.
In this example, the pipeline passes all documents in the zipcodes collection through the following steps:
• the $group operator collects all documents and creates new documents for every combination of the city and
state fields in the source document. A city can have more than one zip code associated with it as different
sections of the city can each have a different zip code.
After this stage in the pipeline, the documents resemble the following:
{
"_id" : {
"state" : "CO",
"city" : "EDGEWATER"
},
"pop" : 13154
}
• the second $group operator collects documents by the state field and use the $avg expression to compute
a value for the avgCityPop field.
The final output of this aggregation operation is:
{
"_id" : "MN",
"avgCityPop" : 5335
},
Return Largest and Smallest Cities by State
To return the smallest and largest cities by population for each state, use the following aggregation operation:
21

db.zipcodes.aggregate( { $group:
{ _id: { state: "$state", city: "$city" },
pop: { $sum: "$pop" } } },
{ $sort: { pop: 1 } },
{ $group:
{ _id : "$_id.state",
biggestCity: { $last: "$_id.city" },
biggestPop: { $last: "$pop" },
smallestCity: { $first: "$_id.city" },
smallestPop: { $first: "$pop" } } },
// the following $project is optional, and
// modifies the output format.
{ $project:
{ _id: 0,
state: "$_id",
biggestCity: { name: "$biggestCity", pop: "$biggestPop" },
smallestCity: { name: "$smallestCity", pop: "$smallestPop" } } } )
Aggregation operations using the aggregate() helper process all documents in the zipcodes collection.
aggregate() combines a number of pipeline (page 7) operators that define the aggregation process.
All documents from the zipcodes collection pass into the pipeline, which consists of the following steps:
• the $group operator collects all documents and creates new documents for every combination of the city and
state fields in the source documents.
By specifying the value of _id as a sub-document that contains both fields, the operation preserves the state
field for use later in the pipeline. The documents produced by this stage of the pipeline have a second field,
pop, which uses the $sum operator to provide the total of the pop fields in the source document.
At this stage in the pipeline, the documents resemble the following:
{
"_id" : {
"state" : "CO",
"city" : "EDGEWATER"
},
"pop" : 13154
}
• $sort operator orders the documents in the pipeline based on the value of the pop field from largest to smallest.
This operation does not alter the documents.
• the second $group operator collects the documents in the pipeline by the state field, which is a field inside
the nested _id document.
Within each per-state document this $group operator specifies four fields: Using the $last expression, the
$group operator creates the biggestcity and biggestpop fields that store the city with the largest pop-ulation
and that population. Using the $first expression, the $group operator creates the smallestcity
and smallestpop fields that store the city with the smallest population and that population.
The documents, at this stage in the pipeline resemble the following:
{
"_id" : "WA",
"biggestCity" : "SEATTLE",
"biggestPop" : 520096,
"smallestCity" : "BENGE",
22

"smallestPop" : 2
}
• The final operation is $project, which renames the _id field to state and moves the biggestCity,
biggestPop, smallestCity, and smallestPop into biggestCity and smallestCity sub-documents.
The output of this aggregation operation is:
{
"state" : "RI",
"biggestCity" : {
"name" : "CRANSTON",
"pop" : 176404
},
"smallestCity" : {
"name" : "CLAYVILLE",
"pop" : 45
}
}
3.2 Aggregation with User Preference Data
Data Model
Consider a hypothetical sports club with a database that contains a users collection that tracks the user’s join dates,
sport preferences, and stores these data in documents that resemble the following:
{
_id : "jane",
joined : ISODate("2011-03-02"),
likes : ["golf", "racquetball"]
}
{
_id : "joe",
likes : ["tennis", "golf", "swimming"]
}
Normalize and Sort Documents
The following operation returns user names in upper case and in alphabetical order. The aggregation includes user
names for all documents in the users collection. You might do this to normalize user names for processing.
db.users.aggregate(
[
{ $project : { name:{$toUpper:"$_id"} , _id:0 } },
{ $sort : { name : 1 } }
]
)
All documents from the users collection pass through the pipeline, which consists of the following operations:
• The $project operator:
– creates a new field called name.
23

– converts the value of the _id to upper case, with the $toUpper operator. Then the $project creates
a new field, named name to hold this value.
– suppresses the id field. $project will pass the _id field by default, unless explicitly suppressed.
• The $sort operator orders the results by the name field.
The results of the aggregation would resemble the following:
{
"name" : "JANE"
},
{
"name" : "JILL"
},
{
"name" : "JOE"
}
Return Usernames Ordered by Join Month
The following aggregation operation returns user names sorted by the month they joined. This kind of aggregation
could help generate membership renewal notices.
db.users.aggregate(
[
{ $project :
{
month_joined : { $month : "$joined" },
name : "$_id",
_id : 0
}
},
{ $sort : { month_joined : 1 } }
]
)
The pipeline passes all documents in the users collection through the following operations:
• The $project operator:
– Creates two new fields: month_joined and name.
– Suppresses the id from the results. The aggregate() method includes the _id, unless explicitly
suppressed.
• The $month operator converts the values of the joined field to integer representations of the month. Then
the $project operator assigns those values to the month_joined field.
• The $sort operator sorts the results by the month_joined field.
The operation returns results that resemble the following:
{
"month_joined" : 1,
"name" : "ruth"
},
{
"month_joined" : 1,
"name" : "harold"
},
24

{
"month_joined" : 1,
"name" : "kate"
}
{
"month_joined" : 2,
"name" : "jill"
}
Return Total Number of Joins per Month
The following operation shows how many people joined each month of the year. You might use this aggregated data
for recruiting and marketing strategies.
db.users.aggregate(
[
{ $project : { month_joined : { $month : "$joined" } } } ,
{ $group : { _id : {month_joined:"$month_joined"} , number : { $sum : 1 } } },
{ $sort : { "_id.month_joined" : 1 } }
]
)
The pipeline passes all documents in the users collection through the following operations:
• The $project operator creates a new field called month_joined.
• The $month operator converts the values of the joined field to integer representations of the month. Then
the $project operator assigns the values to the month_joined field.
• The $group operator collects all documents with a given month_joined value and counts how many docu-ments
there are for that value. Specifically, for each unique value, $group creates a new “per-month” document
with two fields:
– _id, which contains a nested document with the month_joined field and its value.
– number, which is a generated field. The $sum operator increments this field by 1 for every document
containing the given month_joined value.
• The $sort operator sorts the documents created by $group according to the contents of the month_joined
field.
The result of this aggregation operation would resemble the following:
{
"_id" : {
"month_joined" : 1
},
"number" : 3
},
{
"_id" : {
"month_joined" : 2
},
"number" : 9
},
{
"_id" : {
"month_joined" : 3
},
25

"number" : 5
}
Return the Five Most Common “Likes”
The following aggregation collects top five most “liked” activities in the data set. This type of analysis could help
inform planning and future development.
db.users.aggregate(
[
{ $unwind : "$likes" },
{ $group : { _id : "$likes" , number : { $sum : 1 } } },
{ $sort : { number : -1 } },
{ $limit : 5 }
]
)
The pipeline begins with all documents in the users collection, and passes these documents through the following
operations:
• The $unwind operator separates each value in the likes array, and creates a new version of the source
document for every element in the array.
Example
Given the following document from the users collection:
{
_id : "jane",
likes : ["golf", "racquetball"]
}
The $unwind operator would create the following documents:
{
_id : "jane",
likes : "golf"
}
{
_id : "jane",
likes : "racquetball"
}
• The $group operator collects all documents the same value for the likes field and counts each grouping.
With this information, $group creates a new document with two fields:
– _id, which contains the likes value.
– number, which is a generated field. The $sum operator increments this field by 1 for every document
containing the given likes value.
• The $sort operator sorts these documents by the number field in reverse order.
• The $limit operator only includes the first 5 result documents.
The results of aggregation would resemble the following:
26

{
"_id" : "golf",
"number" : 33
},
{
"_id" : "racquetball",
"number" : 31
},
{
"_id" : "swimming",
"number" : 24
},
{
"_id" : "handball",
"number" : 19
},
{
"_id" : "tennis",
"number" : 18
}
3.3 Map-Reduce Examples
In the mongo shell, the db.collection.mapReduce() method is a wrapper around the mapReduce command.
The following examples use the db.collection.mapReduce() method:
Consider the following map-reduce operations on a collection orders that contains documents of the following
prototype:
{
_id: ObjectId("50a8240b927d5d8b5891743c"),
cust_id: "abc123",
ord_date: new Date("Oct 04, 2012"),
status: 'A',
price: 25,
items: [ { sku: "mmm", qty: 5, price: 2.5 },
{ sku: "nnn", qty: 5, price: 2.5 } ]
}
Return the Total Price Per Customer
Perform the map-reduce operation on the orders collection to group by the cust_id, and calculate the sum of the
price for each cust_id:
1. Define the map function to process each input document:
• In the function, this refers to the document that the map-reduce operation is processing.
• The function maps the price to the cust_id for each document and emits the cust_id and price
pair.
var mapFunction1 = function() {
emit(this.cust_id, this.price);
};
2. Define the corresponding reduce function with two arguments keyCustId and valuesPrices:
27

• The valuesPrices is an array whose elements are the price values emitted by the map function and
grouped by keyCustId.
• The function reduces the valuesPrice array to the sum of its elements.
var reduceFunction1 = function(keyCustId, valuesPrices) {
return Array.sum(valuesPrices);
};
3. Perform the map-reduce on all documents in the orders collection using the mapFunction1 map function
and the reduceFunction1 reduce function.
db.orders.mapReduce(
mapFunction1,
reduceFunction1,
{ out: "map_reduce_example" }
)
This operation outputs the results to a collection named map_reduce_example. If the
map_reduce_example collection already exists, the operation will replace the contents with the re-sults
of this map-reduce operation:
Calculate Order and Total Quantity with Average Quantity Per Item
In this example, you will perform a map-reduce operation on the orders collection for all documents that have
an ord_date value greater than 01/01/2012. The operation groups by the item.sku field, and calculates the
number of orders and the total quantity ordered for each sku. The operation concludes by calculating the average
quantity per order for each sku value:
1. Define the map function to process each input document:
• In the function, this refers to the document that the map-reduce operation is processing.
• For each item, the function associates the sku with a new object value that contains the count of 1
and the item qty for the order and emits the sku and value pair.
var mapFunction2 = function() {
for (var idx = 0; idx < this.items.length; idx++) {
var key = this.items[idx].sku;
var value = {
count: 1,
qty: this.items[idx].qty
};
emit(key, value);
}
};
2. Define the corresponding reduce function with two arguments keySKU and countObjVals:
• countObjVals is an array whose elements are the objects mapped to the grouped keySKU values
passed by map function to the reducer function.
• The function reduces the countObjVals array to a single object reducedValue that contains the
count and the qty fields.
• In reducedVal, the count field contains the sum of the count fields from the individual array ele-ments,
and the qty field contains the sum of the qty fields from the individual array elements.
var reduceFunction2 = function(keySKU, countObjVals) {
reducedVal = { count: 0, qty: 0 };
28

for (var idx = 0; idx < countObjVals.length; idx++) {
reducedVal.count += countObjVals[idx].count;
reducedVal.qty += countObjVals[idx].qty;
}
return reducedVal;
};
3. Define a finalize function with two arguments key and reducedVal. The function modifies the
reducedVal object to add a computed field named avg and returns the modified object:
var finalizeFunction2 = function (key, reducedVal) {
reducedVal.avg = reducedVal.qty/reducedVal.count;
return reducedVal;
};
4. Perform the map-reduce operation on the orders collection using the mapFunction2,
reduceFunction2, and finalizeFunction2 functions.
db.orders.mapReduce( mapFunction2,
reduceFunction2,
{
out: { merge: "map_reduce_example" },
query: { ord_date:
{ $gt: new Date('01/01/2012') }
},
finalize: finalizeFunction2
}
)
This operation uses the query field to select only those documents with ord_date greater than new
Date(01/01/2012). Then it output the results to a collection map_reduce_example. If the
map_reduce_example collection already exists, the operation will merge the existing contents with the
results of this map-reduce operation.
3.4 Perform Incremental Map-Reduce
Map-reduce operations can handle complex aggregation tasks. To perform map-reduce operations, MongoDB provides
the mapReduce command and, in the mongo shell, the db.collection.mapReduce() wrapper method.
If the map-reduce data set is constantly growing, you may want to perform an incremental map-reduce rather than
performing the map-reduce operation over the entire data set each time.
To perform incremental map-reduce:
1. Run a map-reduce job over the current collection and output the result to a separate collection.
2. When you have more data to process, run subsequent map-reduce job with:
• the query parameter that specifies conditions that match only the new documents.
• the out parameter that specifies the reduce action to merge the new results into the existing output
collection.
Consider the following example where you schedule a map-reduce operation on a sessions collection to run at the
end of each day.
29

Data Setup
The sessions collection contains documents that log users’ sessions each day, for example:
db.sessions.save( { userid: "a", ts: ISODate('2011-11-03 14:17:00'), length: 95 } );
db.sessions.save( { userid: "b", ts: ISODate('2011-11-03 14:23:00'), length: 110 } );
db.sessions.save( { userid: "c", ts: ISODate('2011-11-03 15:02:00'), length: 120 } );
db.sessions.save( { userid: "d", ts: ISODate('2011-11-03 16:45:00'), length: 45 } );
Initial Map-Reduce of Current Collection
Run the first map-reduce operation as follows:
1. Define the map function that maps the userid to an object that contains the fields userid, total_time,
count, and avg_time:
var mapFunction = function() {
var key = this.userid;
var value = {
userid: this.userid,
total_time: this.length,
count: 1,
avg_time: 0
};
emit( key, value );
};
2. Define the corresponding reduce function with two arguments key and values to calculate the total time and
the count. The key corresponds to the userid, and the values is an array whose elements corresponds to
the individual objects mapped to the userid in the mapFunction.
var reduceFunction = function(key, values) {
var reducedObject = {
userid: key,
total_time: 0,
count:0,
avg_time:0
};
values.forEach( function(value) {
reducedObject.total_time += value.total_time;
reducedObject.count += value.count;
}
);
return reducedObject;
};
3. Define the finalize function with two arguments key and reducedValue. The function modifies the
reducedValue document to add another field average and returns the modified document.
30

status: 'A',
price: 250,
items: [ { sku: "mmm", qty: 5, price: 2.5 },
{ sku: "nnn", qty: 5, price: 2.5 } ]
}
1. Define the map function that maps the price to the cust_id for each document and emits the cust_id and
price pair:
var map = function() {
emit(this.cust_id, this.price);
};
2. Define the emit function to print the key and value:
var emit = function(key, value) {
print("emit");
print("key: " + key + " value: " + tojson(value));
}
3. Invoke the map function with a single document from the orders collection:
var myDoc = db.orders.findOne( { _id: ObjectId("50a8240b927d5d8b5891743c") } );
map.apply(myDoc);
4. Verify the key and value pair is as you expected.
emit
key: abc123 value:250
5. Invoke the map function with multiple documents from the orders collection:
var myCursor = db.orders.find( { cust_id: "abc123" } );
while (myCursor.hasNext()) {
var doc = myCursor.next();
print ("document _id= " + tojson(doc._id));
map.apply(doc);
print();
}
6. Verify the key and value pairs are as you expected.
See also:
The map function must meet various requirements. For a list of all the requirements for the map function, see
mapReduce, or the mongo shell helper method db.collection.mapReduce().
3.6 Troubleshoot the Reduce Function
The reduce function is a JavaScript function that “reduces” to a single object all the values associated with a par-ticular
key during a map-reduce (page 10) operation. The reduce function must meet various requirements. This
tutorial helps verify that the reduce function meets the following criteria:
• The reduce function must return an object whose type must be identical to the type of the value emitted by
the map function.
• The order of the elements in the valuesArray should not affect the output of the reduce function.
• The reduce function must be idempotent.
32

For a list of all the requirements for the reduce function, see mapReduce, or the mongo shell helper method
db.collection.mapReduce().
Confirm Output Type
You can test that the reduce function returns a value that is the same type as the value emitted from the map function.
1. Define a reduceFunction1 function that takes the arguments keyCustId and valuesPrices.
valuesPrices is an array of integers:
var reduceFunction1 = function(keyCustId, valuesPrices) {
return Array.sum(valuesPrices);
};
2. Define a sample array of integers:
var myTestValues = [ 5, 5, 10 ];
3. Invoke the reduceFunction1 with myTestValues:
reduceFunction1('myKey', myTestValues);
4. Verify the reduceFunction1 returned an integer:
20
5. Define a reduceFunction2 function that takes the arguments keySKU and valuesCountObjects.
valuesCountObjects is an array of documents that contain two fields count and qty:
var reduceFunction2 = function(keySKU, valuesCountObjects) {
reducedValue = { count: 0, qty: 0 };
for (var idx = 0; idx < valuesCountObjects.length; idx++) {
reducedValue.count += valuesCountObjects[idx].count;
reducedValue.qty += valuesCountObjects[idx].qty;
}
};
6. Define a sample array of documents:
var myTestObjects = [
{ count: 1, qty: 5 },
{ count: 2, qty: 10 },
{ count: 3, qty: 15 }
];
7. Invoke the reduceFunction2 with myTestObjects:
reduceFunction2('myKey', myTestObjects);
8. Verify the reduceFunction2 returned a document with exactly the count and the qty field:
{ "count" : 6, "qty" : 30 }
33

Ensure Insensitivity to the Order of Mapped Values
The reduce function takes a key and a values array as its argument. You can test that the result of the reduce
function does not depend on the order of the elements in the values array.
1. Define a sample values1 array and a sample values2 array that only differ in the order of the array elements:
var values1 = [
{ count: 1, qty: 5 },
{ count: 2, qty: 10 },
{ count: 3, qty: 15 }
];
var values2 = [
{ count: 3, qty: 15 },
{ count: 1, qty: 5 },
{ count: 2, qty: 10 }
];
}
};
3. Invoke the reduceFunction2 first with values1 and then with values2:
reduceFunction2('myKey', values1);
reduceFunction2('myKey', values2);
4. Verify the reduceFunction2 returned the same result:
{ "count" : 6, "qty" : 30 }
Ensure Reduce Function Idempotence
Because the map-reduce operation may call a reduce multiple times for the same key, and won’t call a reduce for
single instances of a key in the working set, the reduce function must return a value of the same type as the value
emitted from the map function. You can test that the reduce function process “reduced” values without affecting the
final value.
}
34

};
2. Define a sample key:
var myKey = 'myKey';
3. Define a sample valuesIdempotent array that contains an element that is a call to the reduceFunction2
function:
var valuesIdempotent = [
{ count: 1, qty: 5 },
{ count: 2, qty: 10 },
reduceFunction2(myKey, [ { count:3, qty: 15 } ] )
];
4. Define a sample values1 array that combines the values passed to reduceFunction2:
var values1 = [
{ count: 1, qty: 5 },
{ count: 2, qty: 10 },
{ count: 3, qty: 15 }
];
5. Invoke the reduceFunction2 first with myKey and valuesIdempotent and then with myKey and
values1:
reduceFunction2(myKey, valuesIdempotent);
reduceFunction2(myKey, values1);
6. Verify the reduceFunction2 returned the same result:
{ "count" : 6, "qty" : 30 }
4 Aggregation Reference
Aggregation Pipeline Quick Reference (page 36) Quick reference card for aggregation pipeline.
http://docs.mongodb.org/manualreference/operator/aggregation Aggregation pipeline op-erations
have a collection of operators available to define and manipulate documents in pipeline stages.
Aggregation Commands Comparison (page 40) A comparison of group, mapReduce and aggregate that ex-plores
the strengths and limitations of each aggregation modality.
SQL to Aggregation Mapping Chart (page 42) An overview common aggregation operations in SQL and MongoDB
using the aggregation pipeline and operators in MongoDB and common SQL statements.
Aggregation Interfaces (page 44) The data aggregation interfaces document the invocation format and output for
MongoDB’s aggregation commands and methods.
Variables in Aggregation Expressions (page 44) Use of variables in aggregation pipeline expressions.
35

4.1 Aggregation Pipeline Quick Reference
Stages
Pipeline stages appear in an array. Documents pass through the stages in sequence. All except the $out and
$geoNear stages can appear multiple times in a pipeline.
db.collection.aggregate( [ { <stage> }, ... ] )
Name Description
$geoNearReturns an ordered stream of documents based on the proximity to a geospatial point. Incorporates the
functionality of $match, $sort, and $limit for geospatial data. The output documents include an
additional distance field and can include a location identifier field.
$group Groups input documents by a specified identifier expression and applies the accumulator expression(s),
if specified, to each group. Consumes all input documents and outputs one document per each distinct
group. The output documents only contain the identifier field and, if specified, accumulated fields.
$limit Passes the first n documents unmodified to the pipeline where n is the specified limit. For each input
document, outputs either one document (for the first n documents) or zero documents (after the first n
documents).
$match Filters the document stream to allow only matching documents to pass unmodified into the next
pipeline stage. $match uses standard MongoDB queries. For each input document, outputs either one
document (a match) or zero documents (no match).
$out Writes the resulting documents of the aggregation pipeline to a collection. To use the $out stage, it
must be the last stage in the pipeline.
$projectReshapes each document in the stream, such as by adding new fields or removing existing fields. For
each input document, outputs one document.
$redactReshapes each document in the stream by restricting the content for each document based on
information stored in the documents themselves. Incorporates the functionality of $project and
$match. Can be used to implement field level redaction. For each input document, outputs either one
or zero document.
$skip Skips the first n documents where n is the specified skip number and passes the remaining documents
unmodified to the pipeline. For each input document, outputs either zero documents (for the first n
documents) or one document (if after the first n documents).
$sort Reorders the document stream by a specified sort key. Only the order changes; the documents remain
unmodified. For each input document, outputs one document.
$unwindDeconstructs an array field from the input documents to output a document for each element. Each
output document replaces the array with an element value. For each input document, outputs n
documents where n is the number of array elements and can be zero for an empty array.
Expressions
Expressions can include field paths and system variables (page 36), literals (page 37), expression objects (page 37),
and expression operators (page 37). Expressions can be nested.
Field Path and System Variables
Aggregation expressions use field path to access fields in the input documents. To specify a field path, use a string that
prefixes with a dollar sign $ the field name or the dotted field name, if the field is in embedded document. For example,
"$user" to specify the field path for the user field or "$user.name" to specify the field path to "user.name"
field.
"$<field>" is equivalent to "$$CURRENT.<field>" where the CURRENT (page 45) is a system variable that
defaults to the root of the current object in the most stages, unless stated otherwise in specific stages. CURRENT
36

(page 45) can be rebound.
Along with the CURRENT (page 45) system variable, other system variables (page 44) are also available for use in
expressions. To use user-defined variables, use $let and $map expressions. To access variables in expressions, use
a string that prefixes the variable name with $$.
Literals
Literals can be of any type. However, MongoDB parses string literals that start with a dollar sign $ as a path to a
field and numeric/boolean literals in expression objects (page 37) as projection flags. To avoid parsing literals, use the
$literal expression.
Expression Objects
Expression objects have the following form:
{ <field1>: <expression1>, ... }
If the expressions are numeric or boolean literals, MongoDB treats the literals as projection flags (e.g. 1 or true to
include the field), valid only in the $project stage. To avoid treating numeric or boolean literals as projection flags,
use the $literal expression to wrap the numeric or boolean literals.
Operator Expressions
Operator expressions are similar to functions that take arguments. In general, these expressions take an array of
arguments and have the following form:
{ <operator>: [ <argument1>, <argument2> ... ] }
If operator accepts a single argument, you can omit the outer array designating the argument list:
{ <operator>: <argument> }
To avoid parsing ambiguity if the argument is a literal array, you must wrap the literal array in a $literal expression
or keep the outer array that designates the argument list.
Boolean Expressions Boolean expressions evaluates its argument expressions as booleans and return a boolean as
the result.
In addition to the false boolean value, Boolean expression evaluates as false the following: null, 0, and
undefined values. The Boolean expression evaluates all other values as true, including non-zero numeric values
and arrays.
Name Description
$and Returns true only when all its expressions evaluate to true. Accepts any number of argument
expressions.
$not Returns the boolean value that is the opposite of its argument expression. Accepts a single argument
expression.
$or Returns true when any of its expressions evaluates to true. Accepts any number of argument
expressions.
37

Set Expressions Set expressions performs set operation on arrays, treating arrays as sets. Set expressions ignores
the duplicate entries in each input array and the order of the elements.
If the set operation returns a set, the operation filters out duplicates in the result to output an array that contains only
unique entries. The order of the elements in the output array is unspecified.
If a set contains a nested array element, the set expression does not descend into the nested array but evaluates the
array at top-level.
Name Description
$allElementsRTertuurnes true if no element of a set evaluates to false, otherwise, returns false. Accepts a
single argument expression.
$anyElementTRreutuerns true if any elements of a set evaluate to true; otherwise, returns false. Accepts a
single argument expression.
$setDifferenRceeturns a set with elements that appear in the first set but not in the second set; i.e. performs a
relative complement6 of the second set relative to the first. Accepts exactly two argument
expressions.
$setEquals Returns true if the input sets have the same distinct elements. Accepts two or more argument
expressions.
$setIntersecRteituornns a set with elements that appear in all of the input sets. Accepts any number of argument
expressions.
$setIsSubsetReturns true if all elements of the first set appear in the second set, including when the first set
equals the second set; i.e. not a strict subset7. Accepts exactly two argument expressions.
$setUnion Returns a set with elements that appear in any of the input sets. Accepts any number of argument
expressions.
Comparison Expressions Comparison expressions return a boolean except for $cmp which returns a number.
The comparison expressions take two argument expressions and compare both value and type, using the specified
BSON comparison order for values of different types.
Name Description
$cmp Returns: 0 if the two values are equivalent, 1 if the first value is greater than the second, and -1 if the
first value is less than the second.
$eq Returns true if the values are equivalent.
$gt Returns true if the first value is greater than the second.
$gte Returns true if the first value is greater than or equal to the second.
$lt Returns true if the first value is less than the second.
$lte Returns true if the first value is less than or equal to the second.
$ne Returns true if the values are not equivalent.
Arithmetic Expressions Arithmetic expressions perform mathematic operations on numbers. Some arithmetic ex-pressions
can also support date arithmetic.
4http://en.wikipedia.org/wiki/Complement_(set_theory)
5http://en.wikipedia.org/wiki/Subset
6http://en.wikipedia.org/wiki/Complement_(set_theory)
7http://en.wikipedia.org/wiki/Subset
38

Name Description
$add Adds numbers to return the sum, or adds numbers and a date to return a new date. If adding numbers
and a date, treats the numbers as milliseconds. Accepts any number of argument expressions, but at
most, one expression can resolve to a date.
$divide Returns the result of dividing the first number by the second. Accepts two argument expressions.
$mod Returns the remainder of the first number divided by the second. Accepts two argument expressions.
$multiply Multiplies numbers to return the product. Accepts any number of argument expressions.
$subtract Returns the result of subtracting the second value from the first. If the two values are numbers, return
the difference. If the two values are dates, return the difference in milliseconds. If the two values are a
date and a number in milliseconds, return the resulting date. Accepts two argument expressions. If the
two values are a date and a number, specify the date argument first as it is not meaningful to subtract a
date from a number.
String Expressions String expressions, with the exception of $concat, only have a well-defined behavior for
strings of ASCII characters.
$concat behavior is well-defined regardless of the characters used.
Name Description
$concat Concatenates any number of strings.
$strcasecPmeprforms case-insensitive string comparison and returns: 0 if two strings are equivalent, 1 if the first
string is greater than the second, and -1 if the first string is less than the second.
$substr Returns a substring of a string, starting at a specified index position up to a specified length. Accepts
three expressions as arguments: the first argument must resolve to a string, and the second and third
arguments must resolve to integers.
$toLower Converts a string to lowercase. Accepts a single argument expression.
$toUpper Converts a string to uppercase. Accepts a single argument expression.
Text Search Expressions
Name Description
$meta Access text search metadata.
Array Expressions
Name Description
$size Returns the number of elements in the array. Accepts a single expression as argument.
Variable Expressions
Name Description
$let Defines variables for use within the scope of a subexpression and returns the result of the subexpression.
Accepts named parameters.
$map Applies a subexpression to each element of an array and returns the array of resulting values in order.
Accepts named parameters.
Literal Expressions
Name Description
$literalReturn a value without parsing. Use for values that the aggregation pipeline may interpret as an
expression. For example, use a $literal expression to a string that starts with a $ to avoid parsing a field path.
39

Date Expressions
Name Description
$dayOfMonthReturns the day of the month for a date as a number between 1 and 31.
$dayOfWeek Returns the day of the week for a date as a number between 1 (Sunday) and 7 (Saturday).
$dayOfYear Returns the day of the year for a date as a number between 1 and 366 (leap year).
$hour Returns the hour for a date as a number between 0 and 23.
$millisecond Returns the milliseconds of a date as a number between 0 and 999.
$minute Returns the minute for a date as a number between 0 and 59.
$month Returns the month for a date as a number between 1 (January) and 12 (December).
$second Returns the seconds for a date as a number between 0 and 60 (leap seconds).
$week Returns the week number for a date as a number between 0 (the partial week that precedes the first
Sunday of the year) and 53 (leap year).
$year Returns the year for a date as a number (e.g. 2014).
Conditional Expressions
Name Description
$cond A ternary operator that evaluates one expression, and depending on the result, returns the value of the other two expressions. Accepts either three expressions in an ordered list or three named parameters.
$ifNullReturns either the non-null result of the first expression or the result of the second expression if the expression results in a null result. Null result encompasses instances of undefined values or missing
fields. Accepts two expressions as arguments. The result of the second expression can be null.
Accumulators
Accumulators, available only for the $group stage, compute values by combining documents that share the same
group key. Accumulators take as input a single expression, evaluating the expression once for each input document,
and maintain their state for the group of documents.
Name Description
$addToSet Returns an array of unique expression values for each group. Order of the array elements is
undefined.
$avg Returns an average for each group. Ignores non-numeric values.
$first Returns a value from the first document for each group. Order is only defined if the documents are
in a defined order.
$last Returns a value from the last document for each group. Order is only defined if the documents are
in a defined order.
$max Returns the highest expression value for each group.
$min Returns the lowest expression value for each group.
$push Returns an array of expression values for each group.
$sum Returns a sum for each group. Ignores non-numeric values.
4.2 Aggregation Commands Comparison
The following table provides a brief overview of the features of the MongoDB aggregation commands.
40

aggregate mapReduce group
De-scrip-tion
New in version 2.2.
Implements the Map-Reduce
Designed with specific goals of
aggregation for processing large
improving performance and
data sets.
usability for aggregation tasks.
Uses a “pipeline” approach
where objects are transformed as
they pass through a series of
pipeline operators such as
$group, $match, and $sort.
See
http://docs.mongodb.org/manualreference/operator/aggregation
for more information on the
pipeline operators.
Provides grouping functionality.
Is slower than the aggregate
command and has less
functionality than the
mapReduce command.
Key
Fea-tures
Pipeline operators can be
repeated as needed.
Pipeline operators need not
produce one output document for
every input document.
Can also generate new
documents or filter out
documents.
In addition to grouping
operations, can perform complex
aggregation tasks as well as
perform incremental aggregation
on continuously growing
datasets.
See Map-Reduce Examples
(page 27) and Perform
Incremental Map-Reduce
(page 29).
Can either group by existing
fields or with a custom keyf
JavaScript function, can group by
calculated fields.
See group for information and
example using the keyf
function.
Flex-i-bil-ity
Limited to the operators and
Custom map, reduce and
expressions supported by the
finalize JavaScript functions
aggregation pipeline.
offer flexibility to aggregation
However, can add computed
logic.
fields, create new virtual
See mapReduce for details and
sub-objects, and extract
restrictions on the functions.
sub-fields into the top-level of
results by using the $project
pipeline operator.
See $project for more
information as well as
http://docs.mongodb.org/manualreference/operator/aggregation
for more information on all the
available pipeline operators.
Custom reduce and
finalize JavaScript functions
offer flexibility to grouping logic.
See group for details and
restrictions on these functions.
Out-put
Re-sults
Returns results in various options
(inline as a document that
contains the result set, a cursor to
the result set) or stores the results
in a collection.
The result is subject to the BSON
Document size limit if returned
inline as a document that
contains the result set.
Changed in version 2.6: Can
return results as a cursor or store
the results to a collection.
Returns results in various options
(inline, new collection, merge,
replace, reduce). See
mapReduce for details on the
output options.
Changed in version 2.2: Provides
much better support for sharded
map-reduce output than previous
versions.
Returns results inline as an array
of grouped items.
The result set must fit within the
maximum BSON document size
limit.
Changed in version 2.2: The
returned array can contain at
most 20,000 elements; i.e. at
most 20,000 unique groupings.
Previous versions had a limit of
10,000 elements.
Shard-ing
Supports non-sharded and
sharded input collections.
Supports non-sharded and
sharded input collections.
Does not support sharded
collection.
Notes Prior to 2.4, JavaScript code
Prior to 2.4, JavaScript code
More
In-for-ma-tion
See Aggregation Pipeline
(page 7) and aggregate.
See Map-Reduce (page 10) and
mapReduce.
See group.
41

4.3 SQL to Aggregation Mapping Chart
The aggregation pipeline (page 7) allows MongoDB to provide native aggregation capabilities that corresponds to
many common data aggregation operations in SQL.
The following table provides an overview of common SQL aggregation terms, functions, and concepts and the corre-sponding
MongoDB aggregation operators:
SQL Terms,
Functions, and
Concepts
MongoDB Aggregation Operators
WHERE $match
GROUP BY $group
HAVING $match
SELECT $project
ORDER BY $sort
LIMIT $limit
SUM() $sum
COUNT() $sum
join No direct corresponding operator; however, the $unwind operator allows for
somewhat similar functionality, but with fields embedded within the document.
Examples
The following table presents a quick reference of SQL aggregation statements and the corresponding MongoDB state-ments.
The examples in the table assume the following conditions:
• The SQL examples assume two tables, orders and order_lineitem that join by the
order_lineitem.order_id and the orders.id columns.
• The MongoDB examples assume one collection orders that contain documents of the following prototype:
{
cust_id: "abc123",
ord_date: ISODate("2012-11-02T17:04:11.102Z"),
status: 'A',
price: 50,
items: [ { sku: "xxx", qty: 25, price: 1 },
{ sku: "yyy", qty: 25, price: 1 } ]
}
42

SQL Example MongoDB Example Description
SELECT COUNT(*) AS count
db.orders.aggregate( [
FROM orders
{
$group: {
_id: null,
count: { $sum: 1 }
}
}
] )
Count all records from orders
SELECT SUM(price) AS total
FROM orders
{
$group: {
_id: null,
total: { $sum: "$price" }
}
}
] )
Sum the price field from orders
SELECT cust_id,
SUM(price) AS total
FROM orders
GROUP BY cust_id
{
$group: {
_id: "$cust_id",
}
}
] )
For each unique cust_id, sum the
price field.
SELECT cust_id,
SUM(price) AS total
FROM orders
GROUP BY cust_id
ORDER BY total
{
$group: {
_id: "$cust_id",
}
},
{ $sort: { total: 1 } }
] )
For each unique cust_id, sum the
price field, results sorted by sum.
SELECT cust_id,
ord_date,
SUM(price) AS total
FROM orders
GROUP BY cust_id,
ord_date
{
$group: {
_id: {
cust_id: "$cust_id",
ord_date: {
month: { $month: "$ord_date" },
day: { $dayOfMonth: "$ord_date" },
year: { $year: "$ord_date"}
}
},
}
}
] )
For each unique cust_id,
ord_date grouping, sum the
price field. Excludes the time
portion of the date.
SELECT cust_id,
count(*)
FROM orders
{
$group: {
For cust_id with multiple records,
return the cust_id and the corre-sponding
record count.
43

4.4 Aggregation Interfaces
Aggregation Commands
Name Description
aggregate Performs aggregation tasks (page 7) such as group using the aggregation framework.
count Counts the number of documents in a collection.
distinct Displays the distinct values found for a specified key in a collection.
group Groups documents in a collection by the specified key and performs simple aggregation.
mapReduce Performs map-reduce (page 10) aggregation for large data sets.
Aggregation Methods
Name Description
db.collection.aggregate()Provides access to the aggregation pipeline (page 7).
db.collection.group() Groups documents in a collection by the specified key and performs simple
aggregation.
db.collection.mapReduce()Performs map-reduce (page 10) aggregation for large data sets.
4.5 Variables in Aggregation Expressions
Aggregation expressions (page 36) can use both user-defined and system variables.
Variables can hold any BSON type data. To access the value of the variable, use a string with the variable name
prefixed with double dollar signs ($$).
If the variable references an object, to access a specific field in the object, use the dot notation; i.e.
"$$<variable>.<field>".
User Variables
User variable names can contain the ascii characters [_a-zA-Z0-9] and any non-ascii character.
User variable names must begin with a lowercase ascii letter [a-z] or a non-ascii character.
System Variables
MongoDB offers the following system variables:
44

Variable Description
ROOT
References the root document, i.e. the top-level doc-ument,
currently being processed in the aggregation
pipeline stage.
CURRENT
References the start of the field path being processed
in the aggregation pipeline stage. Unless documented
otherwise, all stages start with CURRENT (page 45) the
same as ROOT (page 45).
CURRENT (page 45) is modifiable. However, since
$<field> is equivalent to $$CURRENT.<field>,
rebinding CURRENT (page 45) changes the meaning of
$ accesses.
DESCEND
One of the allowed results of a $redact expression.
PRUNE One of the allowed results of a $redact expression.
KEEP One of the allowed results of a $redact expression.
See also:
$let, $redact
45

Index
C
CURRENT (system variable available in aggregation), 45
D
DESCEND (system variable available in aggregation), 45
K
KEEP (system variable available in aggregation), 45
P
PRUNE (system variable available in aggregation), 45
R
ROOT (system variable available in aggregation), 45
46

Mongo db aggregation-guide

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