{"title":"多对多关联","version":"3.0","locale":"zh-hans","docname":"topics/db/examples/many_to_many","url":"/zh-hans/3.0/topics/db/examples/many_to_many/","canonical":"https://djangodocs.dev/zh-hans/3.0/topics/db/examples/many_to_many/","summary":"使用：class:~django.db.models.ManyToManyField 来定义多对多关系","html":"

多对多关联Link to this heading#

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

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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）：

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Python
from django.db import models\n\nclass Publication(models.Model):\n    title = models.CharField(max_length=30)\n\n    class Meta:\n        ordering = ['title']\n\n    def __str__(self):\n        return self.title\n\nclass Article(models.Model):\n    headline = models.CharField(max_length=100)\n    publications = models.ManyToManyField(Publication)\n\n    class Meta:\n        ordering = ['headline']\n\n    def __str__(self):\n        return self.headline\n
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下面是可以使用PythonAPI工具执行的操作示例。

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创建几个“公共出版物”：

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Python console
>>> p1 = Publication(title='The Python Journal')\n>>> p1.save()\n>>> p2 = Publication(title='Science News')\n>>> p2.save()\n>>> p3 = Publication(title='Science Weekly')\n>>> p3.save()\n
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创建一个“Article（报刊上的文章-具体的出版物）”：

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Python console
>>> a1 = Article(headline='Django lets you build Web apps easily')\n
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“公开出版物”被保存前，你不能关联它。

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Python console
>>> a1.publications.add(p1)\nTraceback (most recent call last):\n...\nValueError: "<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.\n
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保存它！

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Python console
>>> a1.save()\n
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用一个 Publication``来关联 ``Article:

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Python console
>>> a1.publications.add(p1)\n
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创建另一个``Article``, 并且设置它的``Publications``

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Python console
>>> a2 = Article(headline='NASA uses Python')\n>>> a2.save()\n>>> a2.publications.add(p1, p2)\n>>> a2.publications.add(p3)\n
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二次添加是OK的，但它不会重复这一关系。

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Python console
>>> a2.publications.add(p3)\n
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添加错误类型的对象会引发：exc：`TypeError‘：

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Python console
>>> a2.publications.add(a1)\nTraceback (most recent call last):\n...\nTypeError: 'Publication' instance expected\n
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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”

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Python console
>>> new_publication = a2.publications.create(title='Highlights for Children')\n
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Article 对象可以访问于它们相关的 Publication 对象：

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Python console
>>> a1.publications.all()\n<QuerySet [<Publication: The Python Journal>]>\n>>> a2.publications.all()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n
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Publication 对象可以访问于它们相关的 Article 对象：

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Python console
>>> p2.article_set.all()\n<QuerySet [<Article: NASA uses Python>]>\n>>> p1.article_set.all()\n<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>\n>>> Publication.objects.get(id=4).article_set.all()\n<QuerySet [<Article: NASA uses Python>]>\n
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可通过 跨关联查询 查询多对多关联:

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Python console
>>> Article.objects.filter(publications__id=1)\n<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications__pk=1)\n<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications=1)\n<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications=p1)\n<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>\n\n>>> Article.objects.filter(publications__title__startswith="Science")\n<QuerySet [<Article: NASA uses Python>, <Article: NASA uses Python>]>\n\n>>> Article.objects.filter(publications__title__startswith="Science").distinct()\n<QuerySet [<Article: NASA uses Python>]>\n
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count() 函数也可以配合 distinct() 。

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Python console
>>> Article.objects.filter(publications__title__startswith="Science").count()\n2\n\n>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()\n1\n\n>>> Article.objects.filter(publications__in=[1,2]).distinct()\n<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications__in=[p1,p2]).distinct()\n<QuerySet [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]>\n
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支持反向m2m查询（比如，从没有 ManyToManyField 的表开始）：

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Python console
>>> Publication.objects.filter(id=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(pk=1)\n<QuerySet [<Publication: The Python Journal>]>\n\n>>> Publication.objects.filter(article__headline__startswith="NASA")\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n\n>>> Publication.objects.filter(article__id=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(article__pk=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(article=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(article=a1)\n<QuerySet [<Publication: The Python Journal>]>\n\n>>> Publication.objects.filter(article__in=[1,2]).distinct()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n>>> Publication.objects.filter(article__in=[a1,a2]).distinct()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n
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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）：

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Python console
>>> Article.objects.exclude(publications=p2)\n<QuerySet [<Article: Django lets you build Web apps easily>]>\n
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如果我们删除 Publication， 它的 Articles 无法访问它：

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Python console
>>> p1.delete()\n>>> Publication.objects.all()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]>\n>>> a1 = Article.objects.get(pk=1)\n>>> a1.publications.all()\n<QuerySet []>\n
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如果我们删除了 Article，它的 Publications 也无法访问它：

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Python console
>>> a2.delete()\n>>> Article.objects.all()\n<QuerySet [<Article: Django lets you build Web apps easily>]>\n>>> p2.article_set.all()\n<QuerySet []>\n
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通过m2m的另一端添加：

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Python console
>>> a4 = Article(headline='NASA finds intelligent life on Earth')\n>>> a4.save()\n>>> p2.article_set.add(a4)\n>>> p2.article_set.all()\n<QuerySet [<Article: NASA finds intelligent life on Earth>]>\n>>> a4.publications.all()\n<QuerySet [<Publication: Science News>]>\n
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使用关键字通过另一端添加：

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Python console
>>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')\n>>> p2.article_set.all()\n<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>\n>>> a5 = p2.article_set.all()[1]\n>>> a5.publications.all()\n<QuerySet [<Publication: Science News>]>\n
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从 Article 中移除 Publication：

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Python console
>>> a4.publications.remove(p2)\n>>> p2.article_set.all()\n<QuerySet [<Article: Oxygen-free diet works wonders>]>\n>>> a4.publications.all()\n<QuerySet []>\n
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从另一端移除：

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Python console
>>> p2.article_set.remove(a5)\n>>> p2.article_set.all()\n<QuerySet []>\n>>> a5.publications.all()\n<QuerySet []>\n
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可以设置关系：

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Python console
>>> a4.publications.all()\n<QuerySet [<Publication: Science News>]>\n>>> a4.publications.set([p3])\n>>> a4.publications.all()\n<QuerySet [<Publication: Science Weekly>]>\n
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关系可以被清除：

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Python console
>>> p2.article_set.clear()\n>>> p2.article_set.all()\n<QuerySet []>\n
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可以从另一端清除：

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Python console
>>> p2.article_set.add(a4, a5)\n>>> p2.article_set.all()\n<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>\n>>> a4.publications.all()\n<QuerySet [<Publication: Science News>, <Publication: Science Weekly>]>\n>>> a4.publications.clear()\n>>> a4.publications.all()\n<QuerySet []>\n>>> p2.article_set.all()\n<QuerySet [<Article: Oxygen-free diet works wonders>]>\n
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重新创建我们刚删除的 Article 和 Publication ：

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Python console
>>> p1 = Publication(title='The Python Journal')\n>>> p1.save()\n>>> a2 = Article(headline='NASA uses Python')\n>>> a2.save()\n>>> a2.publications.add(p1, p2, p3)\n
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批量删除 Publications - 引用已经删除的出版者应该是：

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Python console
>>> Publication.objects.filter(title__startswith='Science').delete()\n>>> Publication.objects.all()\n<QuerySet [<Publication: Highlights for Children>, <Publication: The Python Journal>]>\n>>> Article.objects.all()\n<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>]>\n>>> a2.publications.all()\n<QuerySet [<Publication: The Python Journal>]>\n
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批量删除文章：

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Python console
>>> q = Article.objects.filter(headline__startswith='Django')\n>>> print(q)\n<QuerySet [<Article: Django lets you build Web apps easily>]>\n>>> q.delete()\n
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After the delete(), the\nQuerySet cache needs to be cleared, and the\nreferenced objects should be gone:

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