Many-to-many relationalships
这篇是 Django 文档中的一些例子，在 tutorial 中没有全部展示的东西看例子很容易就明白了，这里我就大概翻译一下，也作为自己的一个笔记。
models.py
多对多关系，是在模型中用 ManyToManyField 定义。
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',) # 这里是单元素元组请注意，有一个逗号
class Article(models.Model):
headline = models.CharField(max_length = 100)
publications = models.ManyToManyField(Publication)
def __str__(self):
return self.headline
class Meta:
ordering = ('headline',) # 同样是单元素元组请注意逗号
Article 是一张表，有 headline 和 publication 两个字段，publication 字段对应 Publication 这张表，多对多关系，One Article can have mutiple Publication and one Publication can have Mutiple Publication。下面就是演示 python manage.py shell 中的 API 用法：
python manage.py shell API 用法
实例化 Publication 对象，save() 存到数据库中
>>> p1 = Publication(title = 'The Python Journel')
>>> p1.save()
>>> p2 = Publication(title = 'Science News')
>>> p2.save()
>>> p3 = Publication(title = 'Science Weekly')
>>> p3.save()
实例化 Article，与 Publicaton 联系起来
>>> a1 = Article(headline = "Django let's you build Web apps easily")
>>> a1.save()
>>> a1.piblications.add(p1)
这里把前面实例化的三个 Publication 全部加到 a2 中了
>>> a2 = Article('headlinde = "NASA use Python")
>>> a2.save()
>>> a2.publications.add(p1, p2)
>>> a2.publications.add(p3)
重复添加也没有问题
>>> a.publication.add(p3)
实例化 Article 的同时添加 Publication
>>> new_publication = a2.publication.create(title = "Highlings for Children")
查看每个 Article 对应的 Publication
>>> a1.publications.all()
[<Publication: The Python Journal>]
>>> a2.publications.all()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]
也可通过 Publication 关联的 Article 查看，QuerySet 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>]
然后就是各种 API，增删改查，没什么多说的，摸索两下就知道怎么用了
>>> Article.objects.filter(id = 1)
>>> Article.objects.filter(pk = 1)
>>> Article.objects.filter(publications=1)
>>> Article.objects.filter(publications=p1)