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PHP Manual :: MongoCollection::group

MongoCollection::getWriteConcern || MongoCollection::insert || MongoCollection || PHP Manual

(PECL mongo >=0.9.2)

MongoCollection::groupPerforms an operation similar to SQL's GROUP BY command


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



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 value of the aggregation counter object.


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


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.

  • "maxTimeMS"

    Specifies a cumulative time limit in milliseconds for processing the operation on the server (does not include idle time). If the operation is not completed by the server within the timeout period, a MongoExecutionTimeoutException will be thrown.

Return Values

Returns an array containing the result.


Version Description
1.5.0 Added "maxTimeMS" option.
1.2.11 Emits E_DEPRECATED when options is scalar.


Example #1 MongoCollection::group() example

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


->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(; }";

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



The above example will output something similar to:


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.


->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
$condition = array('condition' => array("a" => array( '$gt' => 1)));

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



The above example will output something similar to:

array(4) {
  array(1) {
    array(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.


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


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