Reference with parentheses about django-locallibrary-tutorial HOT 4 CLOSED

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 a genre can contain many books and a Book can cover many genres.
    # Genre class has already been defined so we can specify the object above.
    language = models.ForeignKey('Language', on_delete=models.SET_NULL, null=True)

language = models.ForeignKey(Language, on_delete=models.SET_NULL, null=True)