[edit] Last updated: Fri, 25 May 2012

MongoCollection::group

(PECL mongo >=0.9.2)

MongoCollection::group — Performs an operation similar to SQL's GROUP BY command

Description

public array MongoCollection::group ( mixed $keys , array $initial , MongoCode $reduce [, array $options = array() ] )

Parameters

keys

Fields to group by. If an array or non-code object is passed, it will be the key used to group results.

1.0.4+: If keys is an instance of MongoCode, keys will be treated as a function that returns the key to group by (see the "Passing a keys function" example below).

initial

Initial value of the aggregation counter object.

reduce

A function that takes two arguments (the current document and the aggregation to this point) and does the aggregation.

options

Optional parameters to the group command. Valid options include:

"condition"

Criteria for including a document in the aggregation.
"finalize"

Function called once per unique key that takes the final output of the reduce function.

Return Values

Returns an array containing the result.

Examples

Example #1 MongoCollection::group() example

This groups documents by category and creates a list of names within that category.


<?php

$collection->insert(array("category" => "fruit", "name" => "apple"));
$collection->insert(array("category" => "fruit", "name" => "peach"));
$collection->insert(array("category" => "fruit", "name" => "banana"));
$collection->insert(array("category" => "veggie", "name" => "corn"));
$collection->insert(array("category" => "veggie", "name" => "broccoli"));

$keys = array("category" => 1);

$initial = array("items" => array());

$reduce = "function (obj, prev) { prev.items.push(obj.name); }";

$g = $collection->group($keys, $initial, $reduce);

echo json_encode($g['retval']);

?>

The above example will output something similar to:

[{"category":"fruit","items":["apple","peach","banana"]},{"category":"veggie","items":["corn","broccoli"]}]

Example #2 MongoCollection::group() example

This example doesn't use any key, so every document will be its own group. It also uses a condition: only documents that match this condition will be processed by the grouping function.


<?php

$collection->save(array("a" => 2));
$collection->save(array("b" => 5));
$collection->save(array("a" => 1));

// use all fields
$keys = array();

// set intial values
$initial = array("count" => 0);

// JavaScript function to perform
$reduce = "function (obj, prev) { prev.count++; }";

// only use documents where the "a" field is greater than 1
$condition = array("a" => array( '$gt' => 1));

$g = $collection->group($keys, $initial, $reduce, $condition);

var_dump($g);

?>

The above example will output something similar to:

array(4) {
  ["retval"]=>
  array(1) {
    [0]=>
    array(1) {
      ["count"]=>
      float(1)
    }
  }
  ["count"]=>
  float(1)
  ["keys"]=>
  int(1)
  ["ok"]=>
  float(1)
}

Example #3 Passing a keys function

If you want to group by something other than a field name, you can pass a function as the first parameter of MongoCollection::group() and it will be run against each document. The return value of the function will be used as its grouping value.

This example demonstrates grouping by the num field modulo 4.


<?php

$c->group(new MongoCode('function(doc) { return {mod : doc.num % 4}; }'),
     array("count" => 0),
     new MongoCode('function(current, total) { total.count++; }'));

?>

add a note User Contributed Notes MongoCollection::group

nick at pitchinteractive dot com 05-Jan-2012 12:51


Here's my code to do an equivalent of a GROUP BY and a SUM





<?php


$contributionCol = $db->customers->contribution;


$group = $contributionCol->group(array('date' => true), array('sum' => 0), "function (obj, prev) { prev['sum'] += obj.amount; }");


?>





This groups by the 'date' column and sums over the 'amount' column. In my testing this was much slower than querying all rows and doing the Group by code with PHP. It could just be my particular setup and data set.





Also at first my amount column was a string, which caused the results to be concatenated rather than arithmetic addition, something else to watch out for.

Evgeniy Abduzhapparov 12-Nov-2010 05:05


Here I am posting how I get tags from my documents. Documents should have 'tags' field which is array of strings:

{'tags':['php', 'mongo']}

<?php

$keys = array();

$initial = array('tags'=>array(), 'count'=>0);

$reduce = 'function (doc, total) { if (doc.tags.length) {doc.tags.forEach(function (e) {total.tags[e]=total.tags[e]||0; total.tags[e]++; total.count++;});} }';

$criteria = array(

    'condition' => array(

        'tags' => array('$exists'=>true)

    ),

);

?>

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