# Django 2.2.9.dev20191202104323 文档

# 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）：

    from django.db import models

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

        class Meta:
            ordering = ['title']

        def __str__(self):
            return self.title

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

        class Meta:
            ordering = ['headline']

        def __str__(self):
            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”：

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

创建一个“Article（报刊上的文章-具体的出版物）”：

    >>> 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。

    >>> a1.publications.add(p1)
    Traceback (most recent call last):
    ...
    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！

    >>> a1.save()

用一个 `Publication``来关联 ``Article`:

    >>> a1.publications.add(p1)

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

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

二次添加是OK的，但它不会重复这一关系。

    >>> a2.publications.add(p3)

添加错误类型的对象会引发：exc：`TypeError‘：

    >>> a2.publications.add(a1)
    Traceback (most recent call last):
    ...
    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”

    >>> 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：

    >>> a1.publications.all()
    <QuerySet [<Publication: The Python Journal>]>
    >>> a2.publications.all()
    <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：

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

可通过 跨关联查询 查询多对多关联:

    >>> Article.objects.filter(publications__id=1)
    <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
    >>> Article.objects.filter(publications__pk=1)
    <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
    >>> Article.objects.filter(publications=1)
    <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
    >>> Article.objects.filter(publications=p1)
    <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>

    >>> Article.objects.filter(publications__title__startswith="Science")
    <QuerySet [<Article: NASA uses Python>, <Article: NASA uses Python>]>

    >>> Article.objects.filter(publications__title__startswith="Science").distinct()
    <QuerySet [<Article: NASA uses Python>]>

`count()` 函数也可以配合 `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()
    <QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>
    >>> Article.objects.filter(publications__in=[p1,p2]).distinct()
    <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）：

    >>> Publication.objects.filter(id=1)
    <QuerySet [<Publication: The Python Journal>]>
    >>> Publication.objects.filter(pk=1)
    <QuerySet [<Publication: The Python Journal>]>

    >>> Publication.objects.filter(article__headline__startswith="NASA")
    <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>

    >>> Publication.objects.filter(article__id=1)
    <QuerySet [<Publication: The Python Journal>]>
    >>> Publication.objects.filter(article__pk=1)
    <QuerySet [<Publication: The Python Journal>]>
    >>> Publication.objects.filter(article=1)
    <QuerySet [<Publication: The Python Journal>]>
    >>> Publication.objects.filter(article=a1)
    <QuerySet [<Publication: The Python Journal>]>

    >>> Publication.objects.filter(article__in=[1,2]).distinct()
    <QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
    >>> Publication.objects.filter(article__in=[a1,a2]).distinct()
    <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）：

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

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

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

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

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

通过m2m的另一端添加：

    >>> a4 = Article(headline='NASA finds intelligent life on Earth')
    >>> a4.save()
    >>> p2.article_set.add(a4)
    >>> p2.article_set.all()
    <QuerySet [<Article: NASA finds intelligent life on Earth>]>
    >>> a4.publications.all()
    <QuerySet [<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()
    <QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
    >>> a5 = p2.article_set.all()[1]
    >>> a5.publications.all()
    <QuerySet [<Publication: Science News>]>

从 `Article` 中移除 `Publication`：

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

从另一端移除：

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

可以设置关系：

    >>> a4.publications.all()
    <QuerySet [<Publication: Science News>]>
    >>> a4.publications.set([p3])
    >>> a4.publications.all()
    <QuerySet [<Publication: Science Weekly>]>

关系可以被清除：

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

可以从另一端清除：

    >>> p2.article_set.add(a4, a5)
    >>> p2.article_set.all()
    <QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
    >>> a4.publications.all()
    <QuerySet [<Publication: Science News>, <Publication: Science Weekly>]>
    >>> a4.publications.clear()
    >>> a4.publications.all()
    <QuerySet []>
    >>> p2.article_set.all()
    <QuerySet [<Article: Oxygen-free diet works wonders>]>

重新创建我们刚删除的 `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 `Publications` - 引用已经删除的出版者应该是：

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

批量删除文章：

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

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

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