protected function getForeignKey($foreign)
    {
        $on = $this->wrapTable($foreign->on);

        // We need to columnize the columns that the foreign key is being defined for
        // so that it is a properly formatted list. Once we have done this, we can
        // return the foreign key SQL declaration to the calling method for use.
        $columns = $this->columnize($foreign->columns);

        $onColumns = $this->columnize((array) $foreign->references);

        return ", foreign key($columns) references $on($onColumns)";
    }