Python-сообщество

class Book(models.Model):
    #Тут идёт описание полей
    def comments_count(self):
        from comment.models import Comment
        return len(Comment.objects.filter(book=self))

#Импорт стандартных модулей и т.д.
from book.models import Book
class Comment(models.Model):
    #Другие поля
    book = models.ForeignKey(Book, verbose_name='Ссылка на книгу')

class Book(models.Model):
    #Тут идёт описание полей
    def comments_count(self):
        return self.comment_set.all().count()

from comment.models import BookComment
class Book(models.Model):
    title = models.CharField(max_length=300)
    def comments_count(self):
        return BookComment.objects.filter(book=self).count()

class BookComment(models.Model):
    book = models.ForeignKey('book.Book')
    text = models.TextField()

from book.models import Book
class BookComment(models.Model):
    book = models.ForeignKey(Book)
    text = models.TextField()

class Book(models.Model):
    title = models.CharField(max_length=300)
    def comments_count(self):
        return self.comment_set.count()