多对多关联

    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）：

    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”：

    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)
    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”

    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 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 。

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

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