Django的Many-to-Many（多对多）模型

提示：扫一扫查出行
【扫一扫了解最新限行尾号】
复制提示

日期：2012-05-05 ｜来源：未知｜作者：redice ｜人围观｜ 1 人鼓掌了！

鲲鹏Web数据抓取 - 专业Web数据采集服务提供者

参考：《DjangoBook2.0》数据模型高级进阶

经典的例子：一本书有多个作者，一个作者有多本书，典型的多对多关系。

设计模型如下：

from django.db import models
class Author(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=40)
email = models.EmailField()
def __unicode__(self):
return self.name
class Meta:
db_table = "author"
class Book(models.Model):
title = models.CharField(max_length=200)
authors = models.ManyToManyField(Author)
def __unicode__(self):
return self.title
class Meta:
db_table = "book"

python manage.py sqlall books（App名）看一下Django帮我们生成的数据库表结构：

CREATE TABLE `author` (
`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
`first_name` varchar(30) NOT NULL,
`last_name` varchar(40) NOT NULL,
`email` varchar(75) NOT NULL
)
;
CREATE TABLE `book_authors` (
`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
`book_id` integer NOT NULL,
`author_id` integer NOT NULL,
UNIQUE (`book_id`, `author_id`)
)
;
ALTER TABLE `book_authors` ADD CONSTRAINT `author_id_refs_id_22051734` FOREIGN KEY (`author_id`) REFERENCES `author` (`id`);
CREATE TABLE `book` (
`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
`title` varchar(200) NOT NULL
)
;
ALTER TABLE `book_authors` ADD CONSTRAINT `book_id_refs_id_77516490` FOREIGN KEY (`book_id`) REFERENCES `book` (`id`);

book表中并没有authors字段，其中的book_authors表就是用来存放书和作者多对多映射关系的。

访问多值：

一本书的所有作者：

b = Book.objects.get(id=50)

b.authors.all()

b.authors.filter(first_name='Adam')

反向也可以，一个作者的所有书：

a = Author.objects.get(id=1)

a.book_set.all()

给多对多字段添加值（添加多对多关系）：

a = Author.objects.get(id=1)

b = Book.objects.get(id=50)

b.authors.add(a)

从多对多字段中删除值（删除多对多关系）：

a = Author.objects.get(id=1)

b = Book.objects.get(id=50)

b.authors.remove(a) 或者 b.authors.filter(id=1).delete()

我想自定义关系表!!!

为什么？

可能的原因：

1）我想添加一些额外的字段；

2）与现有系统集成，关系表已经存在。

定义方法如下：

# coding: utf-8
from django.db import models
class Author(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=40)
email = models.EmailField()
def __unicode__(self):
return self.name
class Meta:
db_table = "author"
class Book(models.Model):
title = models.CharField(max_length=200)
authors = models.ManyToManyField(Author, through='BookAuthor') # 注意多了through参数
def __unicode__(self):
return self.title
class Meta:
db_table = "book"
class BookAuthor(models.Model):
book = models.ForeignKey(Book)
author = models.ForeignKey(Author)
created_at = models.DateTimeField(auto_now_add=True) # 我要添加额外的字段
class Meta:
db_table = "book_author_relationship" # 我需要用自定义的名称

再看一下此时的数据库表结构：

BEGIN;
CREATE TABLE `author` (
`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
`first_name` varchar(30) NOT NULL,
`last_name` varchar(40) NOT NULL,
`email` varchar(75) NOT NULL
)
;
CREATE TABLE `book` (
`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
`title` varchar(200) NOT NULL
)
;
CREATE TABLE `book_author_relationship` (
`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
`book_id` integer NOT NULL,
`author_id` integer NOT NULL,
`created_at` datetime NOT NULL
)
;
ALTER TABLE `book_author_relationship` ADD CONSTRAINT `author_id_refs_id_6b774382` FOREIGN KEY (`author_id`) REFERENCES `author` (`id`);
ALTER TABLE `book_author_relationship` ADD CONSTRAINT `book_id_refs_id_7cf7763a` FOREIGN KEY (`book_id`) REFERENCES `book` (`id`);
CREATE INDEX `book_author_relationship_752eb95b` ON `book_author_relationship` (`book_id`);
CREATE INDEX `book_author_relationship_337b96ff` ON `book_author_relationship` (`author_id`);
COMMIT;

爽！

但是这样做也是有不便的地方：多对多字段的add()和remove()方法就不能用了。这里有很好的解释：http://stackoverflow.com/questions/1755591/many-to-many-relationships-with-additional-data-on-the-relationship

https://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships

那此时我们如何添加和删除映射关系呢？直接使用BookAuthor模型。

例如：

a = Author.objects.get(id=1)

b = Book.objects.get(id=50)

BookAuthor(book=b, author=a).save()