我有一个产品模型和另一个名为Course的模型，该模型继承了产品，但有一个视频字段和一个作者，作者是一个外键，教师模型继承自用户模型，用户模型继承自AbstractUser
相关型号：

class User(AbstractUser):
    username = models.SlugField(default="", null=False, db_index=True, blank=True)  # forced by django admin problems :(
    password = models.CharField(max_length=255, null=True)
    email = models.EmailField(max_length=255, unique=True)
    group = models.ManyToManyField(Group)
    is_teacher = models.BooleanField(default=False, null=False)
    is_seller = models.BooleanField(default=False, null=False)
    phoneNum = PhoneNumberField(null=False, unique=True, default='')
    USERNAME_FIELD = "email"
    REQUIRED_FIELDS = ["username", "first_name", "last_name", "password"]

    def save(self, *args, **kwargs):
        self.username = slugify(self.first_name + self.last_name)
        super().save(*args, **kwargs)

class Product(models.Model):
    name = models.CharField(max_length=100, null=False, blank=True)
    shoppers = models.ManyToManyField(User, related_name='shopper')
    tumb = models.ImageField(upload_to=course_tumb_directory_path, null=False)
    lastUpdate = models.DateTimeField(auto_now=True)
    price = models.DecimalField(null=False, default=1000000, max_digits=7, decimal_places=0)

class Teacher(User):
    TOPICS = [
        ("BP", "Basic Programming"),
        ("AP", "Advanced Programming"),
        ("CS", "Computer Science"),
        ("MS", "Mathematics"),
        ("CH", "Chemistry"),
        ("BL", "BioLogy"),
        ("PH", "physics"),
        ("EL", "Electronics"),
        ("RG", "Religious"),
        ("Or", "Other"),
    ]
    topic = models.CharField(max_length=2, choices=TOPICS, default=TOPICS[-1][0])

class Course(Product):
    video = models.FileField(upload_to=course_directory_path, null=True,
                             validators=[
                                 FileExtensionValidator(allowed_extensions=['MOV', 'avi', 'mp4', 'webm', 'mkv'])])
    author = models.ForeignKey(Teacher, on_delete=models.CASCADE, null=True)

当我试图进行迁移时，它是这样说的：

It is impossible to add a non-nullable field 'product_ptr' to course without specifying a default. This is because the database needs something to populate existing rows.

我正在使用PostgreSQL作为我的数据库，它不允许我刷新它，所以我放弃了它，并重新创建了一个新的，但我仍然不能使迁移
我尝试了以下答案：You are trying to add a non-nullable field 'new_field' to userprofile without a default
Django: You are trying to add a non-nullable field 'slug' to post without a default; we can't do that
It is impossible to add a non-nullable field 'id' to video without specifying a default
You are trying to add a non-nullable field 'id' to contact_info without a default
Error : "You are trying to add a non-nullable field"
我以前遇到过这种错误，但有一件事我搞不清楚，那就是它与字段无关，所以我无法修复它
Python语言：3.10.6
Django ：4.1
PostgreSQL语言：psql 14.6
操作系统：Ubuntu 22.04（非虚拟机）