多对多关系

使用ManyToManyField来定义多对多关系

在此示例中，可以在多个Publication对象中发布Article，并且Publication有多个Article

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',)

以下是可以使用Python API设施执行的操作的示例。请注意，如果您对多对多关系使用an intermediate model，则会禁用某些相关管理器的方法，因此其中一些示例将不适用于此类模型。

创建几个Publications：

>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> p2 = Publication(title='Science News')
>>> p2.save()
>>> p3 = Publication(title='Science Weekly')
>>> p3.save()

创建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.

保存！

>>> a1.save()

将Article与Publication：

>>> a1.publications.add(p1)

创建另一个Article，并将其设置为在Publications中显示：

>>> a2 = Article(headline='NASA uses Python')
>>> a2.save()
>>> a2.publications.add(p1, p2)
>>> a2.publications.add(p3)

添加2次也没关系：

>>> a2.publications.add(p3)

添加错误类型的对象会引起TypeError：

>>> a2.publications.add(a1)
Traceback (most recent call last):
...
TypeError: 'Publication' instance expected

使用create()在一个步骤中创建Publication并添加到Article：

>>> new_publication = a2.publications.create(title='Highlights for Children')

Article对象可以访问其相关的Publication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>]

Publication对象可以访问其相关的Article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>]

可以使用lookups across relationships查询多对多关系：

>>> Article.objects.filter(publications__id=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()
[<Article: NASA uses Python>]

count()函数也尊重distinct()：

>>> Article.objects.filter(publications__title__startswith="Science").count()
2

>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()
1

>>> 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>]

支持反向m2m查询（即，从不具有ManyToManyField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__startswith="NASA")
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

>>> 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: Science Weekly>, <Publication: The Python Journal>]
>>> Publication.objects.filter(article__in=[a1,a2]).distinct()
[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

排除相关项目的工作原理如你所期望的（虽然涉及的SQL有点复杂）：

>>> Article.objects.exclude(publications=p2)
[<Article: Django lets you build Web apps easily>]

如果我们删除Publication，则其Articles将无法访问：

>>> 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Article，则其Publications将无法访问：

>>> a2.delete()
>>> Article.objects.all()
[<Article: Django lets you build Web apps easily>]
>>> p2.article_set.all()
[]

通过m2m的'other'端添加：

>>> 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>]

使用关键字通过另一端添加：

>>> 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Article中删除Publication：

>>> a4.publications.remove(p2)
>>> p2.article_set.all()
[<Article: Oxygen-free diet works wonders>]
>>> a4.publications.all()
[]

从另一端：

>>> p2.article_set.remove(a5)
>>> p2.article_set.all()
[]
>>> a5.publications.all()
[]

可以分配关系集。分配清除任何现有集成员：

>>> 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()
[]

你可以从另一端清除：

>>> 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>]

重新创建我们已删除的Article和Publication：

>>> 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 - 对已删除出版物的引用应该是：

>>> Publication.objects.filter(title__startswith='Science').delete()
>>> Publication.objects.all()
[<Publication: Highlights for Children>, <Publication: The Python Journal>]
>>> 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 - 引用删除的对象应该去：

>>> q = Article.objects.filter(headline__startswith='Django')
>>> print(q)
[<Article: Django lets you build Web apps easily>]
>>> q.delete()

在delete()之后，需要清除QuerySet缓存，并且引用的对象应该消失：

>>> print(q)
[]
>>> p1.article_set.all()
[<Article: NASA uses Python>]

调用clear()的替代方法是分配空集：

>>> 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()
[]