Python3.x Django 模型类方法使用错误的 `self`

当我导航到此模型贷款的管理页面或通用视图时，我收到以下错误:

File "library_app/catalog/models.py", line 118, in __str__
return f'{self.book.title} ({self.id})'
AttributeError: 'NoneType' object has no attribute 'title'

我设法找到了我的BookInstance类的__str__函数.

class Loan(models.Model):
book_instance = models.OneToOneField(BookInstance, on_delete=models.RESTRICT)
user = models.ForeignKey(User, on_delete=models.RESTRICT)
date = models.DateField(
    default=django_timezone.now,
    help_text='Date borrowed'
)
due_back = models.DateField(
    default= django_timezone.now() + datetime.timedelta(30), 
    help_text='Date due to be returned'
)
returned_date = models.DateField(null=True, blank=True, help_text='Date book was returned')



class BookInstance(models.Model):
id = models.UUIDField(primary_key=True, default=uuid.uuid4)
book = models.ForeignKey(Book, on_delete=models.RESTRICT, null=True)

LOAN_STATUS = (
    ('m', 'Maintence'),
    ('o', 'On loan'),
    ('a', 'Available'),
    ('r', 'Reserved'),
)

status = models.CharField(
    max_length=1,
    choices=LOAN_STATUS,
    blank=True,
    default='m',
    help_text='Book Availability'
)

def __str__(self):
    return f'{self.book.title} ({self.id})'

我猜在这些页面中，当__str__方法被调用时，使用的是错误的self变量.我不知道如何解决这个问题，所以任何提示都将不胜感激.