class Book(models.Model):
    author = models.ForeignKey('Author', related_name='authors', on_delete=models.PROTECT)
    title = models.CharField(max_length=255)
    price = models.IntegerField()

    def __str__(self):
        return self.title


class Author(models.Model):
    fio = models.CharField(max_length=255)
    books = models.ManyToManyField(Book, related_name='books', blank=True)

    def __str__(self):
        return self.fio