function on($expr)
    {
        $lastJoin = $this->getLastJoinSource();

        // Join all given expressions with AND
        if ($lastJoin->right instanceof On) {
            // Merge the new ON Expressions with the 
            // old if there exists an ON Expression
            $lastJoin->right->expression = $expr;
        } else {
            // Otherwise create a new ON Expression
            $lastJoin->right = new On($expr);
        }
        return $this;
    }