Any way to get the sum of a column with xPDO?

<?xml version="1.0" encoding="UTF-8"?>
<model package="eventManager" baseClass="xPDOObject" platform="mysql" defaultEngine="MyISAM" version="1.1" phpdoc-package="eventManager" phpdoc-subpackage="model">
	<object class="Events" table="em_events" extends="xPDOObject">
		<field key="eventid" dbtype="int" precision="11" phptype="integer" null="false" index="pk" generated="native" />
		<field key="title" dbtype="varchar" precision="200" phptype="string" null="false" />
		<field key="description" dbtype="varchar" precision="2000" phptype="string" null="false" />
		<field key="date" dbtype="int" precision="15" phptype="integer" null="false" />
		<field key="capacity" dbtype="int" precision="10" phptype="integer" null="true" />
		<field key="last_signup" dbtype="int" precision="10" phptype="integer" null="true" />
		<index alias="PRIMARY" name="PRIMARY" primary="true" unique="true">
			<column key="eventid" collation="A" null="false" />
		</index>  
		<composite alias="Reservations" class="Reservations" local="eventid" foreign="eventid" cardinality="many" owner="local" />
	</object>
	<object class="Reservations" table="em_reservations" extends="xPDOObject">
		<field key="reservationid" dbtype="int" precision="11" phptype="integer" null="false" index="pk" generated="native" />
		<field key="eventid" dbtype="int" precision="10" phptype="integer" null="false" />
		<field key="tickets" dbtype="int" precision="10" phptype="integer" null="false" />
		<field key="firstname" dbtype="varchar" precision="200" phptype="string" null="false" />
		<field key="lastname" dbtype="varchar" precision="200" phptype="string" null="false" />
		<field key="time" dbtype="int" precision="15" phptype="integer" null="false" />
		<field key="address" dbtype="varchar" precision="500" phptype="string" null="false" />
		<field key="email" dbtype="varchar" precision="200" phptype="string" null="false" />
		<field key="remarks" dbtype="varchar" precision="2000" phptype="string" null="false" />
		<field key="phone" dbtype="varchar" precision="30" phptype="string" null="false" />
		<index alias="PRIMARY" name="PRIMARY" primary="true" unique="true">
			<column key="reservationid" collation="A" null="false" />
		</index>
		<composite alias="Events" class="Events" local="eventid" foreign="eventid" cardinality="one" owner="foreign" />
	</object>
</model>

<?php
$class='Reservations';

$c = $modx->newQuery($class);
$c->select('sum(tickets) as cnt');
$c->where(array('eventid'=>$eventid));
$object = $modx->getObject($class, $c);

echo $object->get('cnt');

<?php
$c = $modx->newQuery('Reservations');
$c->select('sum(tickets) as cnt');
$c->where(array('eventid'=>$eventid));
if ($c->prepare() && $c->stmt->execute()) {
    echo $c->stmt->fetchColumn();
}