多对多关联

使用：class:~django.db.models.ManyToManyField 来定义多对多关系

在这个例子中，一篇“Article（报刊上的文章）”可能在多个“公开发行物（对象objects）”中发布，并且一个“公开发行物（对象objects）”也有多个具体发行的对象（Article）：

  1. from django.db import models
  3. class Publication(models.Model):
  4.     title = models.CharField(max_length=30)
  6.     class Meta:
  7.         ordering = ['title']
  9.     def __str__(self):
  10.         return self.title
  12. class Article(models.Model):
  13.     headline = models.CharField(max_length=100)
  14.     publications = models.ManyToManyField(Publication)
  16.     class Meta:
  17.         ordering = ['headline']
  19.     def __str__(self):
  20.         return self.headline

下面是可以使用PythonAPI工具执行的操作示例。

创建几个“公共出版物”：

  1. >>> p1 = Publication(title='The Python Journal')
  2. >>> p1.save()
  3. >>> p2 = Publication(title='Science News')
  4. >>> p2.save()
  5. >>> p3 = Publication(title='Science Weekly')
  6. >>> p3.save()

创建一个“Article（报刊上的文章-具体的出版物）”：

  1. >>> a1 = Article(headline='Django lets you build web apps easily')

“公开出版物”被保存前，你不能关联它。

  1. >>> a1.publications.add(p1)
  2. Traceback (most recent call last):
  3. ...
  4. ValueError: "<Article: Django lets you build web apps easily>" needs to have a value for field "id" before this many-to-many relationship can be used.

保存它！

  1. >>> a1.save()

用一个 Publication``来关联 ``Article:

  1. >>> a1.publications.add(p1)

创建另一个``Article``, 并且设置它的``Publications``

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

二次添加是OK的，但它不会重复这一关系。

  1. >>> a2.publications.add(p3)

添加错误类型的对象会引发：exc：`TypeError‘：

  1. >>> a2.publications.add(a1)
  2. Traceback (most recent call last):
  3. ...
  4. TypeError: 'Publication' instance expected

使用：meth：~django.db.models.fields.related.RelatedManager.create::一步一步地创建和添加一个“Publication”到一个“Article”

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

Article 对象可以访问于它们相关的 Publication 对象：

  1. >>> a1.publications.all()
  2. <QuerySet [<Publication: The Python Journal>]>
  3. >>> a2.publications.all()
  4. <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>

Publication 对象可以访问于它们相关的 Article 对象：

  1. >>> p2.article_set.all()
  2. <QuerySet [<Article: NASA uses Python>]>
  3. >>> p1.article_set.all()
  4. <QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
  5. >>> Publication.objects.get(id=4).article_set.all()
  6. <QuerySet [<Article: NASA uses Python>]>

可通过 跨关联查询 查询多对多关联:

  1. >>> Article.objects.filter(publications__id=1)
  2. <QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
  3. >>> Article.objects.filter(publications__pk=1)
  4. <QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
  5. >>> Article.objects.filter(publications=1)
  6. <QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
  7. >>> Article.objects.filter(publications=p1)
  8. <QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
  10. >>> Article.objects.filter(publications__title__startswith="Science")
  11. <QuerySet [<Article: NASA uses Python>, <Article: NASA uses Python>]>
  13. >>> Article.objects.filter(publications__title__startswith="Science").distinct()
  14. <QuerySet [<Article: NASA uses Python>]>

count() 函数也可以配合 distinct() 。

  1. >>> Article.objects.filter(publications__title__startswith="Science").count()
  2. 2
  4. >>> Article.objects.filter(publications__title__startswith="Science").distinct().count()
  5. 1
  7. >>> Article.objects.filter(publications__in=[1,2]).distinct()
  8. <QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
  9. >>> Article.objects.filter(publications__in=[p1,p2]).distinct()
  10. <QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>

支持反向m2m查询（比如，从没有 ManyToManyField 的表开始）：

  1. >>> Publication.objects.filter(id=1)
  2. <QuerySet [<Publication: The Python Journal>]>
  3. >>> Publication.objects.filter(pk=1)
  4. <QuerySet [<Publication: The Python Journal>]>
  6. >>> Publication.objects.filter(article__headline__startswith="NASA")
  7. <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
  9. >>> Publication.objects.filter(article__id=1)
  10. <QuerySet [<Publication: The Python Journal>]>
  11. >>> Publication.objects.filter(article__pk=1)
  12. <QuerySet [<Publication: The Python Journal>]>
  13. >>> Publication.objects.filter(article=1)
  14. <QuerySet [<Publication: The Python Journal>]>
  15. >>> Publication.objects.filter(article=a1)
  16. <QuerySet [<Publication: The Python Journal>]>
  18. >>> Publication.objects.filter(article__in=[1,2]).distinct()
  19. <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
  20. >>> Publication.objects.filter(article__in=[a1,a2]).distinct()
  21. <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>

排除相关条目的工作方式正如你所期望的那样（尽管相关 SQL 有点复杂）：

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

如果我们删除 Publication， 它的 Articles 无法访问它：

  1. >>> p1.delete()
  2. >>> Publication.objects.all()
  3. <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]>
  4. >>> a1 = Article.objects.get(pk=1)
  5. >>> a1.publications.all()
  6. <QuerySet []>

如果我们删除了 Article，它的 Publications 也无法访问它：

  1. >>> a2.delete()
  2. >>> Article.objects.all()
  3. <QuerySet [<Article: Django lets you build web apps easily>]>
  4. >>> p2.article_set.all()
  5. <QuerySet []>

通过m2m的另一端添加：

  1. >>> a4 = Article(headline='NASA finds intelligent life on Earth')
  2. >>> a4.save()
  3. >>> p2.article_set.add(a4)
  4. >>> p2.article_set.all()
  5. <QuerySet [<Article: NASA finds intelligent life on Earth>]>
  6. >>> a4.publications.all()
  7. <QuerySet [<Publication: Science News>]>

使用关键字通过另一端添加：

  1. >>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')
  2. >>> p2.article_set.all()
  3. <QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
  4. >>> a5 = p2.article_set.all()[1]
  5. >>> a5.publications.all()
  6. <QuerySet [<Publication: Science News>]>

从 Article 中移除 Publication：

  1. >>> a4.publications.remove(p2)
  2. >>> p2.article_set.all()
  3. <QuerySet [<Article: Oxygen-free diet works wonders>]>
  4. >>> a4.publications.all()
  5. <QuerySet []>

从另一端移除：

  1. >>> p2.article_set.remove(a5)
  2. >>> p2.article_set.all()
  3. <QuerySet []>
  4. >>> a5.publications.all()
  5. <QuerySet []>

可以设置关系：

  1. >>> a4.publications.all()
  2. <QuerySet [<Publication: Science News>]>
  3. >>> a4.publications.set([p3])
  4. >>> a4.publications.all()
  5. <QuerySet [<Publication: Science Weekly>]>

关系可以被清除：

  1. >>> p2.article_set.clear()
  2. >>> p2.article_set.all()
  3. <QuerySet []>

可以从另一端清除：

  1. >>> p2.article_set.add(a4, a5)
  2. >>> p2.article_set.all()
  3. <QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
  4. >>> a4.publications.all()
  5. <QuerySet [<Publication: Science News>, <Publication: Science Weekly>]>
  6. >>> a4.publications.clear()
  7. >>> a4.publications.all()
  8. <QuerySet []>
  9. >>> p2.article_set.all()
  10. <QuerySet [<Article: Oxygen-free diet works wonders>]>

重新创建我们刚删除的 Article 和 Publication ：

  1. >>> p1 = Publication(title='The Python Journal')
  2. >>> p1.save()
  3. >>> a2 = Article(headline='NASA uses Python')
  4. >>> a2.save()
  5. >>> a2.publications.add(p1, p2, p3)

批量删除 Publications - 引用已经删除的出版者应该是：

  1. >>> Publication.objects.filter(title__startswith='Science').delete()
  2. >>> Publication.objects.all()
  3. <QuerySet [<Publication: Highlights for Children>, <Publication: The Python Journal>]>
  4. >>> Article.objects.all()
  5. <QuerySet [<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>]>
  6. >>> a2.publications.all()
  7. <QuerySet [<Publication: The Python Journal>]>

批量删除文章：

  1. >>> q = Article.objects.filter(headline__startswith='Django')
  2. >>> print(q)
  3. <QuerySet [<Article: Django lets you build web apps easily>]>
  4. >>> q.delete()

After the delete(), the QuerySet cache needs to be cleared, and the referenced objects should be gone:

  1. >>> print(q)
  2. <QuerySet []>
  3. >>> p1.article_set.all()
  4. <QuerySet [<Article: NASA uses Python>]>