django:Many-to-many关系
要定义多对多关系，使用ManyToManyField字。 （注：django版本1.4） 本例中，一篇文章可以被很多出版社发表，而一个出版社也会发表多个文章。
from django.db import models
class Publication(models.Model):
title = models.CharField(max_length=30)
# On Python 3: def __str__(self):
def __unicode__(self):
return self.title
class Meta:
ordering = ('title',)
class Article(models.Model):
headline = models.CharField(max_length=100)
publications = models.ManyToManyField(Publication)
# On Python 3: def __str__(self):
def __unicode__(self):
return self.headline
class Meta:
ordering = ('headline',) 接下来我们使用Python API 功能执行操作的例子。
创建两个出版社：
>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> p2 = Publication(title='Science News')
>>> p2.save()
>>> p3 = Publication(title='Science Weekly')
>>> p3.save() 新建一个文章：
>>> a1 = Article(headline='Django lets you build Web apps easily')只有把它保存了，才能把它和出版社关联在一起。否则会出错如下：
>>> a1.publications.add(p1)
Traceback (most recent call last):
...
ValueError: 'Article' instance needs to have a primary key value before a many-to-many relationship can be used.保存。
>>> a1.save() 关联文章和出版社。
建立第2篇文章，让它在两个出版社中出现。
>>> a2 = Article(headline='NASA uses Python')
>>> a2.save()
>>> a2.publications.add(p1, p2)
>>> a2.publications.add(p3) 再次添加也OK
>>> a2.publications.add(p3) 如果添加错误类型的对象会发生 TypeError：
>>> a2.publications.add(a1)
Traceback (most recent call last):
...
TypeError: 'Publication' instance expected 使用create()一次创建并把出版社指派到一篇文章：
>>> new_publication = a2.publications.create(title='Highlights for Children') 文章对象有权访问和它们相关联的出版社对象（物件）：
>>> a1.publications.all()
[<Publication: The Python Journal>]
>>> a2.publications.all()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>] 出版社对象也有权访问与它们相关联的文章对象：
>>> p2.article_set.all()
[<Article: NASA uses Python>]
>>> p1.article_set.all()
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Publication.objects.get(id=4).article_set.all()
[<Article: NASA uses Python>] 使用 lookups across relationships 来query多对多关系：
>>> Article.objects.filter(publications__id__exact=1)
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__pk=1)
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications=1)
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications=p1)
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__title__startswith="Science")
[<Article: NASA uses Python>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__title__startswith="Science").distinct() count()函数与distinct()表现相同：
>>> Article.objects.filter(publications__title__startswith="Science").count()
2
>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()
1 `这里写代码片`
>>> Article.objects.filter(publications__in=[1,2]).distinct()
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__in=[p1,p2]).distinct()
[<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>] 反向 m2m查询也被支持（如，开始的表格没有 ManyToManyField）：
>>> Publication.objects.filter(id__exact=1) [<Publication: The Python Journal>] >>> Publication.objects.filter(pk=1) [<Publication: The Python Journal>] >>> Publication.objects.filter(article__headline__startswith="NASA") [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>] >>> Publication.objects.filter(article__id__exact=1) [<Publication: The Python Journal>] >>> Publication.objects.filter(article__pk=1) [<Publication: The Python Journal>] >>> Publication.objects.filter(article=1) [<Publication: The Python Journal>] >>> Publication.objects.filter(article=a1) [<Publication: The Python Journal>] >>> Publication.objects.filter(article__in=[1,2]).distinct() [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]<p></p><p>>>> Publication.objects.filter(article__in=[a1,a2]).distinct() [<Publication: Highlights for Children>, <Publication: Science News>, <Publication>, <Publication: Science Weekly>, <Publication: The <a href="http://lib.csdn.net/base/11" class="replace_word" title="Python知识库" target="_blank" style="color:#df3434; font-weight:bold;">Python</a> Journal>]</p> 也可以按自己预期的那样排除一个相关的项目（尽管使用的SQL语句有一点复杂）：
>>> Article.objects.exclude(publications=p2)
[<Article: Django lets you build Web apps easily>] 如果我们删除一个出版社，那么它的文章就不能够被访问：
>>> p1.delete()
>>> Publication.objects.all()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]
>>> a1 = Article.objects.get(pk=1)
>>> a1.publications.all()
[] 如果我们删除一篇文章，则它的出版社也不能访问它：
>>> a2.delete()
>>> Article.objects.all()
[<Article: Django lets you build Web apps easily>]
>>> p2.article_set.all()
[] 经由m2m的另一方法来添加：
>>> a4 = Article(headline='NASA finds intelligent life on Earth')
>>> a4.save()
>>> p2.article_set.add(a4)
>>> p2.article_set.all()
[<Article: NASA finds intelligent life on Earth>]
>>> a4.publications.all()
[<Publication: Science News>] 经由关键字的另一方法添加：
>>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')
>>> p2.article_set.all()
[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]
>>> a5 = p2.article_set.all()[1]
>>> a5.publications.all()
[<Publication: Science News>] 从文章中移除出版社：
>>> a4.publications.remove(p2)
>>> p2.article_set.all()
[<Article: Oxygen-free diet works wonders>]
>>> a4.publications.all()
[] 从出版社中移除文章：
>>> p2.article_set.remove(a5)
>>> p2.article_set.all()
[]
>>> a5.publications.all()
[] 关系集合可以被分配。分配时所有已经存在的集合成员会被清除：
>>> a4.publications.all()
[<Publication: Science News>]
>>> a4.publications = [p3]
>>> a4.publications.all()
[<Publication: Science Weekly>] 关系集合可以清除：
>>> p2.article_set.clear()
>>> p2.article_set.all()
[] 而且你也可以从另一端清除（注：关系的另一端）：
>>> p2.article_set.add(a4, a5)
>>> p2.article_set.all()
[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]
>>> a4.publications.all()
[<Publication: Science News>, <Publication: Science Weekly>]
>>> a4.publications.clear()
>>> a4.publications.all()
[]
>>> p2.article_set.all()
[<Article: Oxygen-free diet works wonders>]重建我们删除过的文章和出版社：
>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> a2 = Article(headline='NASA uses Python')
>>> a2.save()
>>> a2.publications.add(p1, p2, p3) 批量删除一些出版社-引用的被删出版社应当去掉：
>>> Publication.objects.filter(title__startswith='Science').delete()
>>> Publication.objects.all()
>>> Article.objects.all()
[<Article: Django lets you build Web apps easily>, <Article: NASA finds intelligent life on Earth>, <Article: NASA uses Python>,<Article: Oxygen-free diet works wonders>]
>>> a2.publications.all()
[<Publication: The Python Journal>] 批量删除一些文章-引用的被删除对象应当去掉：
>>> q = Article.objects.filter(headline__startswith='Django')
>>> print(q)
[<Article: Django lets you build Web apps easily>]
>>> q.delete() 在delete()以后，QuerySet缓存需要清理，而且引用对象应该被去掉：
>>> print(q)
[]
>>> p1.article_set.all()
[<Article: NASA uses Python>]除了调用clear()之外，可以赋值一个空的集合：
>>> p1.article_set = []
>>> p1.article_set.all()
[]
>>> a2.publications = [p1, new_publication]
>>> a2.publications.all()
[<Publication: Highlights for Children>, <Publication: The Python Journal>]
>>> a2.publications = []
>>> a2.publications.all()
[] 附：
The Django Book中的1个例子。
按部就班照着它做，没看到它写如何添加Book。
model.py类
#coding=utf-8
from django.db import models
class Publisher(models.Model):
name=models.CharField(max_length=30)
address=models.CharField(max_length=50)
city=models.CharField(max_length=60)
state_province=models.CharField(max_length=30)
country=models.CharField(max_length=50)
website=models.URLField()
def __unicode__(self):
return self.name
class Meta:
ordering=['name']
class Author(models.Model):
first_name=models.CharField(max_length=100)
last_name=models.CharField(max_length=40)
email=models.EmailField(max_length=40,verbose_name='email_address',blank=True)
def __unicode__(self):
return u'%s %s' % (self.first_name,self.last_name)
class Book(models.Model):
title=models.CharField(max_length=100)
authors=models.ManyToManyField(Author)
publisher=models.ForeignKey(Publisher)
publication_date=models.DateField()
def __unicode__(self):
return 生成的表是（这里使用mysql）：
表中字段为：
setting.py中installed app要加上mysite.books。
cmd进入命令行，在站点下运行命令：
I:\mysite>manage.py shell >>>from mysite.books.models import Publisher,Author,Book
>>>import datetime
>>>b1=Book(title='Beginer osf Labview',publisher=Publisher.objects.all()[1],publication_date=datetime.datetime.strptime('20130726','%Y%m%d'))
>>>b1.save() #保存一下，得到一个book的ID
>>>b1.authors.add(Author.objects.all()[1])
>>>b1<Book: Beginer osf Labview>>>> Book.objects.all()[<Book: Learn C>, <Book: Learn Python>, <Book: Beginer osf Labview>]b1在新建时，需要把出版社和日期都包含进来。否则会出错：
>>>b2=Book()
>>>b2.save()
IntegrityError: (1048, "Column 'publisher_id' cannot be null") 在Book类中可以看到Publisher是外键，在表中会有一个publisher_id不允许为空。
而对于ManyToMany字段，Book中的Author只能使用add方法来添加，添加之前需要通过save()取得一个book的id。
Add以后，不用再次save，也可以保存到数据库？