django 1.7
定义多对多关系，使用 ManyToManyFiled。
本例中，文章Article可以在多个出版物Publication上发表，而出版物可以有多个文章对象。
from django.db import models
class Publication(models.Model):
title = models.CharField(max_length=30)
def __str__(self):
return self.title
class Meta:
ordering = ('title',)
class Article(models.Model):
headline = models.CharField(max_length=100)
publications = models.ManyToManyField(Publication)
def __str__(self):
return self.headline
class Meta:
ordering = ('headline',)
下面是使用Python API执行例子的操作。注意，若你为多对多关系使用了中间体模型，有些关系管理器的方法不能使用，因此这些例子中某些在这种模型下不能工作。
创建几个出版物：
>>> p1 = Publication(title=‘The Python Journal‘)
>>> p1.save()
>>> p2 = Publication(title=‘Science News‘)
>>> p2.save()
>>> p3 = Publication(title=‘Science wekkly‘)
创建文章：
>>> a1 = Article(headline=‘Django lets you build web apps easily‘)
在保存它以前，不能与一个Publication关联。
>>> a1.publications.add(p1)
...
ValueError: ‘Article‘ instance needs to have a primary key value before a many-to-many relationship can be used.
保存
>>> a1.save()
把Article关联到Publication:
>>> a1.publications.add(p1)
创建另一个Article，设定它在两个Publications中出现：
>>> a2 = Article(headline=‘NASA uses Python‘)
>>> a2.save()
>>> a2.publications.add(p1, p2)
>>> a2.publications.add(p3)
第2次添加也是正确的：
>>> a2.publications.add(p3)
添加一个错误类型的对象会产生一个 TypeError：
>>> a2.publictaions.add(a1)
Traceback (most recent call last):
...
TypeError: ‘Publication‘ instance excepted
使用create()一步创建Publications并把它添加到一个文章中：
>>> new_publication = a2.publications.create(title=‘Highlights for Children‘)
Article对象可以访问它们关联的Publication对象：
>>> a1.publications.all()
[<Publication: HIghlights for Children>, ...]
>>> a2.publications.all()
[<Publication: Highlights for Children>, <Publication: Science News>, ... ]
Publication 对象可以访问它们关联的Article对象：
>>> p2.article_set.all()
[<Article: NASA uses Python>]
>>> p1.article_set.all()
[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]
>>> Publication.objects.get(id=4).article_set.all()
[<Article: NASA uses Python>]
可以使用跨关系查询来获得多对多关系查询：
>>> Article.objects.filter(publications__id=1)
[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__pk=1)
[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications=1):
[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications=p1)
[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__title__startwith=‘Science‘)
[<Article: NASA uses Python>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__title__startwith=‘Science‘).distinct()
[<Article: NASA uses Python>]
count()函数也使用distinct()：
>>> Article.objects.filter(publications__title__startwith=‘Science‘).count()
2
>>> Article.objects.filter(publications__title__startwith=‘Science‘).distinct().count()
1
>>> Article.objects.filter(publications__in=[1,2]).distinct()
[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]
>>> Article.objects.filter(publications__in=[p1,p2]).distinct()
[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]
反向m2m查询也被支持（如，从不含ManyToManyField字段的表开始）：
>>> Publication.objects.filter(id=1)
[<Publication: The Python Journal>]
>>> Publication.objects.filter(pk=1)
[<Publication: The Python Journal>]
>>> Publication.objects.filter(article__headline__startwith=‘NASA‘)
[<Publication: Highlights for Children>, <Publication: Science News>, ... ]
>>> Publication.objects.filter(article__id=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.objects.filter(article__in=[a1,a2]).distinct()
[<Publication: Highlights for Children>, <Publication: Science News>, ... ]
排除一个关联项目会按预期工作（尽管使用的SQL有一点复杂）：
>>> Article.objects.exclude(publications=p2)
[<Article: Django lets you build Web apps easily>]
如果我们删除Publication，它的Article将不能访问它：
>>> p1.delete()
>>> Publication.objects.all()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>‘]
>>> a1 = Article.objects.get(pk=1)
>>> a1.publications.all()
[ ]
如果我们删除一个Article，它的Publications将不能访问它：
>>> a2.delete()
>>>Article.objects.all()
[<Artilce: Django lets you bulid Web apps easily>]
>>> p2.article_set.all()
[ ]
通过m2m的另一端进行添加：
>>> a4 = Article(headline=‘NASA finds intelligent lif on Earth‘)
>>>a4.save()
>>>p2.article_set.add(a4)
>>> p2.article_set.all()
[<Article: NASA finds intelligent lif on Earth>]
>>> a4.publications.all()
[<Publication: Science News>]
通过另一端使用关键字添加：
>>> new_article = p2.article_set.create(headline=‘Oxygen-free diet works wonders‘)
>>> p2.article_set.all()
[<Article: NASA finds intelligent lif on Earth>, <Article: Oxygen-free diet works wonders> ]
>>> a5 = p2.article_set.all()[1]
>>> a5.publications.all()
[<Publication: Science News> ]
从1个Article移除Publication：
>>> a4.publications.remove(p2)
>>> p2.article_set.all()
[<Article: Oxygen-free diet works wonders>]
>>> a4.publications.all()
[ ]
而从另一端：
>>> p2.article_set.remove(a5)
>>>p2.article_set.all()
[ ]
>>> a5.publications.all()
[ ]
可以赋值关系集。赋值操作清除任何存在的集合成员：
>>> a4.publications.all()
[<Publication: Science News>]
>>> a4.publications = [p3]
>>> a4.publications.all()
[<Publication: Science Weekly>]
关系集可以清除：
>>> p2.article_set.clear()
>>> p2.article_set.all()
[ ]
也可以从另一端做清除：
>>> p2.article_set.add(a4, a5)
>>> p2.article_set.all()
[<Article: NASA finds intelligent lif on Earth>, <Article: Oxygen-free diet works wonders> ]
>>> a4.publications.clear()
>>> a4.publications.all()
[ ]
>>> p2.article_set.all()
[<Article: Oxygen-free diet works wonders> ]
重新创建我们删除的Article和Publications:
>>> 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Publications - 引用删除的publications也被删除：
>>> Publication.objects.filter(title__startwith=‘Science‘).delete()
>>> Publication.objects.all()
[<Publication: Highlights for Children>, <Publication: The Python Journal>]
>>> Article.objects.all()
[<Article: Django lets you build Web apps easily>, .... ]
>>> a2.publications.all()
[<Publication: The Python Journal>]
批量删除一些article - 引用删除的对象也被删除：
>>> q = Article.objects.filter(headline__startwith=‘Django‘)
>>> print(q)
[<Article: Django lets you build Web apps easily>]
>>> q.delete()
在delete()调用后，QeruySet缓存需要清除，而且音乐的对象也会删掉：
>>> print(q)
[ ]
>>> p1.article_set.all()
[<Article: NASA uses Python>]
调用clear()的替代方法是赋值空集：
>>> 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()
[ ]