class CourseInstructor(models.Model):
    instructor_name = models.ForeignKey(User, on_delete=models.CASCADE)

    def __str__(self):
        return self.instructor_name.username

class Course(models.Model):
    name = models.CharField(max_length=250)

class CourseHour(models.Model):
    from_hour = models.CharField(max_length=4, choices=hours)
    to_hour   = models.CharField(max_length=4, choices=hours)

class Item(models.Model):
    instructor      = models.ForeignKey(CourseInstructor, on_delete=models.DO_NOTHING)
    course          = models.ForeignKey(Course, on_delete=models.CASCADE)
    hours           = models.ForeignKey(CourseHour, on_delete=models.CASCADE)


@receiver(post_save, sender=CourseInstructor)
@receiver(post_save, sender=Course)
@receiver(post_save, sender=CourseHour)
def category_slug_pre_save(sender, instance, created, *args, **kwargs):
    if created:
        Item.objects.create(instructor=instance.instructor_name)
		Item.objects.create(course=instance.name)
		Item.objects.create(hours=[instance.from_hour,instance.to_hour])
	Item.objects.save()