Create and return a string suitable for the rankings
        field when given tuples of choices and rankings.
        Parameters:     ranking_tuples should be an iterable
                            of tuples of form (choice, ranking)

def create_ranking(ranking_tuples):
        """
        Create and return a string suitable for the rankings
        field when given tuples of choices and rankings.
        Parameters:     ranking_tuples should be an iterable
                            of tuples of form (choice, ranking)
        """
        return ",".join([str(r) for c, r in sorted(
                ranking_tuples,
                key=lambda x: x[0].pk
                )])