SUM（）在Mongodb？ 查询问题？

I am trying to write a PHP (Mongo) query from SQL in which I have SUM() but I am not sure my syntax is correct. Can someone enlighten me?

SQL:

$cmd = "SELECT SUM(m_length) FROM pkt_tbl WHERE m_time>=" . $time. " AND m_buffer_latency<=" . $time;

Mongodb Query:

    $find_projection= aggregate(array('$group'=>array('$sum'=>'$m_length')));
    $result = $table -> command($find_projection);

Can I use array_sum is the $result or is there anyway I can use $SUM (Aggregate) in this case. Any help would be appreciated.

Thanks

You should try aggregation

In php this code may help you,

<?php
    $m = new Mongo;
    $c = $m->selectDB("test")->selectCollection("zips");

    $out = $c->aggregate(array(
            '$group' => array(
                '_id' => '$state',
               'totalPop' => array('$sum' => '$pop')
            )
        ),
        array(
            '$match' => array('totalPop' => array('$gte' => 10*1000*1000))
        )
    );

    var_dump($out);
?>

Mongocollection Aggregate

$result = $table->aggregate(
array('$match' => array(
'm_time' => array('$gte' => $max),
'm_buffer_latency' => array('$lte' => $max),
)),
array('$group' => array(
'_id' => true,
'sum_length' => array('$sum' => '$m_length')
))
); 

This answers it all