问对象没有属性“title”

EN

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

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 the file.
    summary = models.Text Field(max_length=1000, help_text="Enter a brief description of the book")
    is bn = models.Char Field('ISBN',max_length=13, help_text='13 Character <a href="https://www.isbn-international.org/content/what-isbn">ISBN number</a>')
    genre = models.Many To Many Field('Genre', help_text="Select a genre for this book")
    # Many To Many Field used because genre can contain many books. Books can cover many genres.
    # Genre class has already been defined so we can specify the object above.

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