class Category(models.Model):
  name_category=models.CharField(max_length=15,verbose_name='Name')
  yes_category=models.NullBooleanField(verbose_name='Name2')
  def __unicode__(self):
    return self.name_category
class CategoryGo(models.Model):
  option=models.ForeignKey(Category)
  name_category=models.CharField(max_length=15)
  yes_category=models.BooleanField()
  def __unicode__(self):
    return self.name_category+'+'+self.option

return self.name_category+ '+'  +self.option if self.option else ""

return self.name_category+ '+'  +self.option.name_category if self.option else ""

return u'{0} {1}'.format(self.name_category, self.option)

print u'{0} {1}'.format("cccd", None)
>>> cccd None