属性错误：＆＃39; NoneType＆＃39;对象没有属性＆＃39; id＆＃39;

Question

我已经达到了Mozilla Django教程的第4章，但后来我遇到了这个错误。我按照它说的所有内容进行了操作，但当我尝试从管理面板中打开 BookInstance 模型时，它给了我这个错误：

  

AttributeError at / admin / catalog / bookinstance /＆＃39; NoneType＆＃39;对象有   没有属性＆＃39; id＆＃39;

以下是我的代码 models.py （我已突出显示发生错误的部分）：

from django.db import models
from django.core.urlresolvers import reverse

class Book(models.Model):
    """
    Model representing a book (but not a specific copy of a book).
    """
    title = models.CharField(max_length=200)
    author = models.ForeignKey('Author', on_delete=models.SET_NULL, null=True)
      # Foreign Key used because book can only have one author, but authors can have multiple books
      # Author as a string rather than object because it hasn't been declared yet in file.
    summary = models.TextField(max_length=1000, help_text="Enter a brief description of the book")
    isbn = models.CharField('ISBN',max_length=13, help_text='13 Character <a href="https://www.isbn-international.org/content/what-isbn">ISBN number</a>')
    genre = models.ManyToManyField(Genre, help_text="Select a genre for this book")
      # ManyToManyField used because Subject can contain many books. Books can cover many subjects.
      # Subject declared as an object because it has already been defined.

    def display_genre(self):
        """
        Creates a string for the Genre. This is required to display genre in Admin.
        """
        return ', '.join([ genre.name for genre in self.genre.all()[:3] ])
        display_genre.short_description = 'Genre'


    def get_absolute_url(self):
        """
        Returns the url to access a particular book instance.
        """
        return reverse('book-detail', args=[str(self.id)])

    def __str__(self):
        """
        String for representing the Model object.
        """
        return self.title



import uuid # Required for unique book instances

class BookInstance(models.Model):
    """
    Model representing a specific copy of a book (i.e. that can be borrowed from the library).
    """
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, help_text="Unique ID for this particular book across whole library")
    book = models.ForeignKey(Book, on_delete=models.SET_NULL, null=True)
    imprint = models.CharField(max_length=200)
    due_back = models.DateField(null=True, blank=True)

    LOAN_STATUS = (
        ('d', 'Maintenance'),
        ('o', 'On loan'),
        ('a', 'Available'),
        ('r', 'Reserved'),
    )

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

    class Meta:
        ordering = ["due_back"]


    def __str__(self):
        """
        String for representing the Model object
        """
        ***return '%s (%s)' %(self.id,self.book.title)***

任何帮助将不胜感激！

编辑：这是完整的错误

编辑：修复它

我用

替换了它

        return str('%s (%s)'% (self.id, self.book.title))

Answer 1

book = models.ForeignKey(Book, on_delete=models.SET_NULL, null=True)您已添加null=True，并在__str__方法中添加了self.book.title，假设self.book是否为无，那么它将无法得到self.book.title。

你也在误导'（'在这里用右括号return '%s (%s)' %(self.id,self.book.title)

添加if条件以检查self.book是否不是。

    def __str__(self):
        """
        String for representing the Model object
        """
        if self.book:
            return '%s (%s)' %(self.id, self.book.title))
        else:
            return '%s (%s)' %(self.id, self.imprint)) # add some other value that you want here

Answer 2

根据你的截图！

在您的 admin.py 中，您可能已将标题提及为 BookInstance 的字段，但未在您的模型定义中定义，因此发生错误。