public function group($group, $context)
{
if (!$group) {
return;
}
if (is_string($group) && strpos($group, 'function') === 0) {
return array('$keyf' => new MongoCode($group));
}
$group = (array) $group;
foreach ($group as $i => $field) {
if (is_int($i)) {
$group[$field] = true;
unset($group[$i]);
}
}
return array('key' => $group);
}