public function addReference($columnName, $refTable, $refColumn,
            $onDelete = 'RESTRICT', $onUpdate = 'RESTRICT') {
        $this->addIndex($columnName);
        $this->newReferences[$columnName] = array(
            'table' => $refTable,
            'column' => $refColumn,
            'onDelete' => $onDelete,
            'onUpdate' => $onUpdate,
        );
        return $this;
    }