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.. _cpp-aggregation: | ||
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==================================== | ||
Transform Your Data with Aggregation | ||
==================================== | ||
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.. facet:: | ||
:name: genre | ||
:values: reference | ||
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.. meta:: | ||
:keywords: code example, transform, computed, pipeline | ||
:description: Learn how to use the C++ driver to perform aggregation operations. | ||
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.. contents:: On this page | ||
:local: | ||
:backlinks: none | ||
:depth: 2 | ||
:class: singlecol | ||
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.. TODO: | ||
.. toctree:: | ||
:titlesonly: | ||
:maxdepth: 1 | ||
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/aggregation/aggregation-tutorials | ||
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Overview | ||
-------- | ||
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In this guide, you can learn how to use the {+driver-short+} to perform | ||
**aggregation operations**. | ||
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Aggregation operations process data in your MongoDB collections and | ||
return computed results. The MongoDB Aggregation framework, which is | ||
part of the Query API, is modeled on the concept of data processing | ||
pipelines. Documents enter a pipeline that contains one or more stages, | ||
and this pipeline transforms the documents into an aggregated result. | ||
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An aggregation operation is similar to a car factory. A car factory has | ||
an assembly line, which contains assembly stations with specialized | ||
tools to do specific jobs, like drills and welders. Raw parts enter the | ||
factory, and then the assembly line transforms and assembles them into a | ||
finished product. | ||
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The **aggregation pipeline** is the assembly line, **aggregation stages** are the | ||
assembly stations, and **operator expressions** are the | ||
specialized tools. | ||
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Aggregation Versus Find Operations | ||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | ||
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You can use find operations to perform the following actions: | ||
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- Select which documents to return | ||
- Select which fields to return | ||
- Sort the results | ||
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You can use aggregation operations to perform the following actions: | ||
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- Run find operations | ||
- Rename fields | ||
- Calculate fields | ||
- Summarize data | ||
- Group values | ||
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Limitations | ||
~~~~~~~~~~~ | ||
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Keep the following limitations in mind when using aggregation operations: | ||
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- Returned documents cannot violate the | ||
:manual:`BSON document size limit </reference/limits/#mongodb-limit-BSON-Document-Size>` | ||
of 16 megabytes. | ||
- Pipeline stages have a memory limit of 100 megabytes by default. You can exceed this | ||
limit by setting the ``allow_disk_use`` field of a ``mongocxx::options::aggregate`` | ||
instance to ``true``. | ||
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.. important:: $graphLookup Exception | ||
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The :manual:`$graphLookup | ||
</reference/operator/aggregation/graphLookup/>` stage has a strict | ||
memory limit of 100 megabytes and ignores the ``allow_disk_use`` field. | ||
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.. _cpp-aggregation-example: | ||
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Aggregation Example | ||
------------------- | ||
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.. note:: | ||
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The examples in this guide use the ``restaurants`` collection in the ``sample_restaurants`` | ||
database from the :atlas:`Atlas sample datasets </sample-data>`. To learn how to create a | ||
free MongoDB Atlas cluster and load the sample datasets, see the :atlas:`Get Started with Atlas | ||
</getting-started>` guide. | ||
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To perform an aggregation, pass a ``mongocxx::pipeline`` instance containing the aggregation | ||
stages to the ``collection.aggregate()`` method. | ||
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The following code example produces a count of the number of bakeries in each borough | ||
of New York. To do so, it uses an aggregation pipeline that contains the following stages: | ||
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- :manual:`$match </reference/operator/aggregation/match/>` stage to filter for documents | ||
in which the ``cuisine`` field contains the value ``"Bakery"`` | ||
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- :manual:`$group </reference/operator/aggregation/group/>` stage to group the matching | ||
documents by the ``borough`` field, accumulating a count of documents for each distinct | ||
value | ||
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.. io-code-block:: | ||
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.. input:: /includes/aggregation.cpp | ||
:start-after: start-match-group | ||
:end-before: end-match-group | ||
:language: cpp | ||
:dedent: | ||
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.. output:: | ||
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{ "_id" : "Brooklyn", "count" : 173 } | ||
{ "_id" : "Queens", "count" : 204 } | ||
{ "_id" : "Bronx", "count" : 71 } | ||
{ "_id" : "Staten Island", "count" : 20 } | ||
{ "_id" : "Missing", "count" : 2 } | ||
{ "_id" : "Manhattan", "count" : 221 } | ||
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Explain an Aggregation | ||
~~~~~~~~~~~~~~~~~~~~~~ | ||
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To view information about how MongoDB executes your operation, you can | ||
instruct the MongoDB query planner to **explain** it. When MongoDB explains | ||
an operation, it returns **execution plans** and performance statistics. | ||
An execution plan is a potential way MongoDB can complete an operation. | ||
When you instruct MongoDB to explain an operation, it returns both the | ||
plan MongoDB executed and any rejected execution plans. | ||
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To explain an aggregation operation, run the ``explain`` database command by specifying | ||
the command in a BSON document and passing it as an argument to the ``run_command()`` | ||
method. | ||
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The following example instructs MongoDB to explain the aggregation operation from the | ||
preceding :ref:`cpp-aggregation-example`: | ||
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.. io-code-block:: | ||
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.. input:: /includes/aggregation.cpp | ||
:start-after: start-explain | ||
:end-before: end-explain | ||
:language: cpp | ||
:dedent: | ||
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.. output:: | ||
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{ "explainVersion" : "2", "queryPlanner" : { "namespace" : "sample_restaurants.restaurants", | ||
"indexFilterSet" : false, "parsedQuery" : { "cuisine" : { "$eq" : "Bakery" } }, "queryHash": | ||
"...", "planCacheKey" : "...", "optimizedPipeline" : true, "maxIndexedOrSolutionsReached": | ||
false, "maxIndexedAndSolutionsReached" : false, "maxScansToExplodeReached" : false, | ||
"winningPlan" : { ... } | ||
... } | ||
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Additional Information | ||
---------------------- | ||
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MongoDB Server Manual | ||
~~~~~~~~~~~~~~~~~~~~~ | ||
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To view a full list of expression operators, see :manual:`Aggregation | ||
Operators. </reference/operator/aggregation/>` | ||
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To learn about assembling an aggregation pipeline and view examples, see | ||
:manual:`Aggregation Pipeline. </core/aggregation-pipeline/>` | ||
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To learn more about creating pipeline stages, see :manual:`Aggregation | ||
Stages. </reference/operator/aggregation-pipeline/>` | ||
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To learn more about explaining MongoDB operations, see | ||
:manual:`Explain Output </reference/explain-results/>` and | ||
:manual:`Query Plans. </core/query-plans/>` | ||
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.. TODO: | ||
Aggregation Tutorials | ||
~~~~~~~~~~~~~~~~~~~~~ | ||
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.. To view step-by-step explanations of common aggregation tasks, see | ||
.. :ref:`cpp-aggregation-tutorials-landing`. | ||
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API Documentation | ||
~~~~~~~~~~~~~~~~~ | ||
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For more information about executing aggregation operations with the {+driver-short+}, | ||
see the following API documentation: | ||
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- `aggregate() <{+api+}/classmongocxx_1_1v__noabi_1_1collection.html#a480f6d0f9986d43b1d17d6ed8876941d>`__ | ||
- `mongocxx::options::aggregate <{+api+}/classmongocxx_1_1v__noabi_1_1options_1_1aggregate.html>`__ |
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.. _cpp-compatibility-tables: | ||
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============= | ||
Compatibility | ||
============= | ||
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.. contents:: On this page | ||
:local: | ||
:backlinks: none | ||
:depth: 2 | ||
:class: singlecol | ||
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.. facet:: | ||
:name: genre | ||
:values: reference | ||
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.. meta:: | ||
:keywords: backwards compatibility, versions, upgrade | ||
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MongoDB Compatibility | ||
--------------------- | ||
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The following compatibility table specifies the recommended version or versions | ||
of the {+driver-long+} for use with a specific version of MongoDB. | ||
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The first column lists the driver version. | ||
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.. sharedinclude:: dbx/lifecycle-schedule-callout.rst | ||
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.. sharedinclude:: dbx/compatibility-table-legend.rst | ||
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.. include:: /includes/mongodb-compatibility-table-cxx.rst | ||
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Language Compatibility | ||
---------------------- | ||
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The following compatibility table specifies the recommended version of the | ||
{+driver-long+} for use with a specific version of C++. | ||
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The first column lists the driver version. | ||
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.. include:: /includes/language-compatibility-table-cxx.rst | ||
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For more information on how to read the compatibility tables, see our guide on | ||
:ref:`MongoDB Compatibility Tables. <about-driver-compatibility>` |
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