118 lines
3.1 KiB
Plaintext
118 lines
3.1 KiB
Plaintext
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Map Reduce
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==========
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The Doctrine MongoDB ODM fully supports the `map reduce`_ functionality via its
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:doc:`Query Builder API <query-builder-api>`.
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.. note::
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From the MongoDB manual:
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Map-reduce is a data processing paradigm for condensing large volumes of
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data into useful aggregated results. In MongoDB, map-reduce operations use
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custom JavaScript functions to map, or associate, values to a key. If a key
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has multiple values mapped to it, the operation reduces the values for the
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key to a single object.
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Imagine a situation where you had an application with a document
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named ``Event`` and it was related to a ``User`` document:
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.. code-block:: php
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<?php
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namespace Documents;
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/** @Document */
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class Event
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{
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/** @Id */
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private $id;
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/** @ReferenceOne(targetDocument="Documents\User") */
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private $user;
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/** @Field(type="string") */
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private $type;
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/** @Field(type="date") */
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private $date;
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/** @Field(type="string") */
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private $description;
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// getters and setters
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}
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/** @Document */
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class User
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{
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// ...
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}
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We may have a situation where we want to run a query that tells us how many
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sales events each user has had. We can easily use the map reduce functionality
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of MongoDB via the ODM's query builder. Here is a simple map reduce example:
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.. code-block:: php
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<?php
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$qb = $dm->createQueryBuilder('Documents\User')
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->field('type')
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->equals('sale')
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->map('function() { emit(this.user.$id, 1); }')
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->reduce('function(k, vals) {
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var sum = 0;
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for (var i in vals) {
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sum += vals[i];
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}
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return sum;
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}');
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$query = $qb->getQuery();
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$results = $query->execute();
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foreach ($results as $user) {
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printf("User %s had %d sale(s).\n", $user['_id'], $user['value']);
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}
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.. note::
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The query builder also has a ``finalize()`` method, which may be used to
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specify a `finalize function`_ to be executed after the reduce step.
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When using map reduce with Doctrine, the results are not hydrated into objects.
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Instead, the raw results are returned directly from MongoDB.
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The preceding example is equivalent to executing the following command via the
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PHP driver directly:
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.. code-block:: php
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<?php
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$db = $mongoClient->selectDB('my_db');
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$map = new MongoCode('function() { emit(this.user.$id, 1); }');
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$reduce = new MongoCode('function(k, vals) {
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var sum = 0;
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for (var i in vals) {
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sum += vals[i];
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}
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return sum;
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}');
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$result = $db->command(array(
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'mapreduce' => 'events',
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'map' => $map,
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'reduce' => $reduce,
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'query' => array('type' => 'sale'),
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));
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foreach ($result['results'] as $user) {
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printf("User %s had %d sale(s).\n", $user['_id'], $user['value']);
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}
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.. _`map reduce`: https://docs.mongodb.com/manual/core/map-reduce/
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.. _`finalize function`: https://docs.mongodb.com/master/reference/command/mapReduce/#mapreduce-finalize-cmd
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