class Course(models.Model):
   title = models.CharField(max_length=100)
   start_date = models.DateField(blank=True, null=True)
class Course_event(models.Model):
   title = models.ForeignKey(Course)
   event_date = models.DateField(blank=True, null=True)

class Course_event(models.Model):
	
	title = models.ForeignKey(Course)
	event_date = models.DateField(blank=True, null=True)
	def save(self, *args, **kwargs):
		course = self.title
		if course.start_date > self.event_date:
			course.start_date = self.event_date
			course.save()
		super(Course_event, self).save(*args, **kwargs)

class Course(models.Model):
    title = models.CharField(max_length=100)
    create_date = models.CharField(blank=True, null=True, auto_now_add=True) # параметр присваивает текущую дату автоматически при создании
    start_date = models.DateField(blank=True, null=True, auto_now=True) # параметр присваивает текущую дату автоматически при каждом сохранении
class Course_event(models.Model):
    title = models.ForeignKey(Course)
    event_date = models.DateField(blank=True, null=True)