# Django 5.2.8 documentation

# 多对多关联

使用 `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。

Create a few `Publication` instances:

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

创建一篇 `Article`：

    >>> a1 = Article(headline="Django lets you build web apps easily")

在保存之前，您无法将其与 `Publication` 相关联：

    >>> 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()

将 `Article` 与 `Publication` 关联起来：

    >>> a1.publications.add(p1)

Create another `Article`, and set it to appear in its publications:

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

添加第二次是可以的，它不会重复关联关系：

    >>> a2.publications.add(p3)

添加错误类型的对象会引发 `TypeError`：

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

使用 `create()` 一步创建并添加一个 `Publication` 到一个 `Article`：

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

`Article` 对象可以访问它们相关联的 `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` 对象可以访问它们相关联的 `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>]>

可以使用 跨关系的查找 来查询多对多关系：

    >>> 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()` 函数也会遵循 `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>]>

支持反向多对多查询（即从没有 `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>]>

排除相关项的操作与您的期望相符，尽管涉及的 SQL 有点复杂：

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

If we delete a `Publication`, its related `Article` instances won't be able to access it:

    >>> 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 []>

If we delete an `Article`, its related `Publication` instances won't be able to access it:

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

通过多对多关系的“另一端”进行添加：

    >>> 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>]>

通过关键字使用多对多关系的“另一端”进行添加：

    >>> 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 []>

关系集合可以被设置：

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

关系集合可以被清除：

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

也可以从另一端清除：

    >>> 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)

Bulk delete some `Publication` instances, and the references to deleted publications will no longer be included in the related entries:

    >>> 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>]>

批量删除一些文章 - 对已删除对象的引用应该被删除：

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

在 `delete()` 后，需要清除 `QuerySet` 缓存，并且引用的对象应该已经消失了：

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