slot = models.OneToOneField(Slot, null=True, blank=True, related_name="presentation")

    title = models.CharField(max_length=100)

    description = MarkupField()

    abstract = MarkupField()

    speaker = models.ForeignKey("speakers.Speaker", related_name="presentations")

    additional_speakers = models.ManyToManyField("speakers.Speaker", blank=True)

    cancelled = models.BooleanField(default=False)

    _proposal = models.OneToOneField(ProposalBase, related_name="presentation")

    section = models.ForeignKey(Section, related_name="presentations")

    @property

    def proposal(self):

        if self._proposal:

            proposal = ProposalBase.objects.get_subclass(pk=self._proposal.pk)

            return proposal

        return None

    def speakers(self):

        yield self.speaker

        for speaker in self.additional_speakers.all():

            yield speaker

    def __unicode__(self):