class Unit(models.Model):
    name = models.CharField(max_length=200)
    start = models.DateTimeField()
    end = models.DateTimeField()
    description = models.TextField()
    deleted = models.BooleanField(default=False)

    def clean(self):
        if self.end and self.start and self.end <= self.start:
            raise ValidationError({
                'end': _('End date should be after start date')
            })

    def get_absolute_url(self):
        return reverse('decentmark:unit_view', kwargs={'unit_id': self.pk})