def rooms(self):

        return Room.objects.filter(schedule=self.day.schedule).order_by("order")

    def __iter__(self):

        times = sorted(set(itertools.chain(*self.slots_qs().values_list("start", "end"))))

        slots = list(self.slots_qs().order_by("start"))

        row = []

        for time, next_time in pairwise(times):

            row = {"time": time, "slots": []}

            for slot in slots:

                if slot.start == time:

                    slot.rowspan = TimeTable.rowspan(times, slot.start, slot.end)

                    row["slots"].append(slot)

            if row["slots"] or next_time is None:

                yield row

    @staticmethod

    def rowspan(times, start, end):

        return times.index(end) - times.index(start)

def pairwise(iterable):

    a, b = itertools.tee(iterable)

    b.next()

    return itertools.izip_longest(a, b)