django设置多对多关系的三种方法

django设置多对多关系的三种方法,如果对第三张表有增加字段的需求，可考虑后两种。

django帮我们生成第三张表

class Author(models.Model):
   name = models.CharField(max_length=32)
   books = models.ManyToManyField('Book')  # 不在Author表中生产字段，生产第三张表

自己创建第三张表

class AuthorBook(models.Model):
   author = models.ForeignKey(Author, on_delete=models.CASCADE)
   book = models.ForeignKey(Book, on_delete=models.CASCADE)
   date = models.DateField()

自建的表和 ManyToManyField 联合使用

使用参数through：将自建的表设置成ManyToManyField的第三张表

class Author(models.Model):
   name = models.CharField(max_length=32)
   books = models.ManyToManyField('Book',through='AuthorBook')

class AuthorBook(models.Model):
   author = models.ForeignKey(Author, on_delete=models.CASCADE)
   book = models.ForeignKey(Book, on_delete=models.CASCADE)
   date = models.DateField()

特殊：自建表中有对同一个表多个外键的情况
使用参数through_fields：指定外键名，说明第三张表中只有这些外键

class Author(models.Model):
   name = models.CharField(max_length=32)
   books = models.ManyToManyField('Book',through='AuthorBook',through_fields=['author','book']

class AuthorBook(models.Model):
   author = models.ForeignKey(Author,related_name='a' ,on_delete=models.CASCADE)
   book = models.ForeignKey(Book, on_delete=models.CASCADE)
   tuijian = models.ForeignKey(Author,related_name='b',on_delete=models.CASCADE)

参数related_name：用来区分同一个表的外键，方便之后用来反向查询