Answered Call to undefined method DBAPI::getFullTableName()

$seats = $_POST["seats"];

$places = $_POST["places"];

$sid = $_POST["sessionID"];

$new_seats = $places - $seats;

// ******** UPDATE SESSION PLACES **********
#################################################################################
require_once('../manager/includes/config.inc.php');
require_once('../manager/includes/document.parser.class.inc.php');
$modx = new DocumentParser;

	$new_seats = update_places($sid, $new_seats);

	function update_places($new_seats) {
        global $modx;
		$table = $modx->db->getFullTableName('xtra_session');
         
		$result = $modx->db->update( 'places = "' . $new_seats . '"', $table, 'id = "' . $sid . '"' );
                return $result;         // Returns 'true' on success, 'false' on failure.
}