多对多

ManyToManyField用来定义多对多关系
多对多在哪个模型中设置ManyToManyField并不重要，在两个模型中任选一个即可 —— 不要两个模型都设置。

在本例中，一篇文章可以在多个发布对象中发布，并且发布具有多个文章对象:

from django.db import models

class Publication(models.Model):
    title = models.CharField(max_length=30)

    def __str__(self):              # __unicode__ on Python 2
        return self.title

    class Meta:
        ordering = ('title',)

class Article(models.Model):
    headline = models.CharField(max_length=100)
    publications = models.ManyToManyField(Publication)

    def __str__(self):              # __unicode__ on Python 2
        return self.headline

    class Meta:
        ordering = ('headline',)

多对多的使用和绑定

# Create a couple of Publications:
>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> p2 = Publication(title='Science News')
>>> p2.save()
>>> p3 = Publication(title='Science Weekly')
>>> p3.save()
#Create an Article:
>>> a1 = Article(headline='Django lets you build Web apps easily')
# You can’t associate it with a Publication until it’s been saved:
>>> 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.

#Save it!
>>> a1.save()
>>> a1.publications.add(p1)
>>> a2 = Article(headline='NASA uses Python')
>>> a2.save()
>>> a2.publications.add(p1, p2)
>>> a2.publications.add(p3)
#可以添加多次,也不会报错
>>> a2.publications.add(p3)

# 但是如果添加绑定的不属于外键关系就会出错
>>> a2.publications.add(a1)
Traceback (most recent call last):
...
TypeError: 'Publication' instance expected

#使用关联对象
>>> a1.publications.all()
[<Publication: The Python Journal>]
>>> a2.publications.all()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]
>>> 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>]

有中间人模式的多对多关系

from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __str__(self):              # __unicode__ on Python 2
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

    def __str__(self):              # __unicode__ on Python 2
        return self.name

class Membership(models.Model):
    person = models.ForeignKey(Person)
    group = models.ForeignKey(Group)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

多对多中间人模式的创建和绑定

#带有中间人的模式不能使用add和create语句创建
#只能先创建对象,然后利用中间人进行绑定,创建对象的时候是不赋值外键字段的,利用中间人绑定之后,会自动创建外键
>>> ringo = Person.objects.create(name="Ringo Starr")
>>> paul = Person.objects.create(name="Paul McCartney")
>>> beatles = Group.objects.create(name="The Beatles")
>>> m1 = Membership(person=ringo, group=beatles,
...     date_joined=date(1962, 8, 16),
...     invite_reason="Needed a new drummer.")
>>> m1.save()
>>> beatles.members.all()
[<Person: Ringo Starr>]
>>> ringo.group_set.all()
[<Group: The Beatles>]
>>> m2 = Membership.objects.create(person=paul, group=beatles,
...     date_joined=date(1960, 8, 1),
...     invite_reason="Wanted to form a band.")
>>> beatles.members.all()
[<Person: Ringo Starr>, <Person: Paul McCartney>]