多对多表结构设计：

from django.db import models
# Create your models here.
class Publisher(models.Model):
    name = models.CharField(max_length=32)
    def __str__(self):
        return self.name

class Book(models.Model):
    name = models.CharField(max_length=32)
    pub = models.ForeignKey(Publisher, on_delete=models.CASCADE, )
    def __str__(self):
        return self.name

    """
    on_delete 在2.0版本之后是必填的
    on_delete= 
        models.CASCADE  级联删除
        PROTECT   保护
        SET(1)    
        SET_DEFAULT  设置为默认值设置为某一个值
        SET_NULL   设置为空
        DO_NOTHING 什么都不变
    """

class Author(models.Model):
    name = models.CharField(max_length=32)
    books = models.ManyToManyField('Book')  # 不会生成某一列 生成一张表

# class AuthorBook(models.Model):
#     author = models.ForeignKey(Author)
#     book = models.ForeignKey(Book)