Called to determine what to tell the clicksorter.
public function clickSort($order) {
if (isset($this->field_alias)) {
// Since fields should always have themselves already added, just
// add a sort on the field.
$params = $this->options['group_type'] != 'group' ? array(
'function' => $this->options['group_type'],
) : array();
$this->query
->addOrderBy(NULL, NULL, $order, $this->field_alias, $params);
}
}