{"title":"多对一关联","version":"3.0","locale":"zh-hans","docname":"topics/db/examples/many_to_one","url":"/zh-hans/3.0/topics/db/examples/many_to_one/","canonical":"https://djangodocs.dev/zh-hans/3.0/topics/db/examples/many_to_one/","summary":"要定义一个多对一关联，使用 ForeignKey. 下面是可以使用PythonAPI工具执行的操作示例。","html":"

多对一关联Link to this heading#

\n

要定义一个多对一关联，使用 ForeignKey:

\n
Code
from django.db import models\n\nclass Reporter(models.Model):\n    first_name = models.CharField(max_length=30)\n    last_name = models.CharField(max_length=30)\n    email = models.EmailField()\n\n    def __str__(self):\n        return "%s %s" % (self.first_name, self.last_name)\n\nclass Article(models.Model):\n    headline = models.CharField(max_length=100)\n    pub_date = models.DateField()\n    reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)\n\n    def __str__(self):\n        return self.headline\n\n    class Meta:\n        ordering = ['headline']\n
\n

下面是可以使用PythonAPI工具执行的操作示例。

\n

创建一些 Reporters：

\n
Python console
>>> r = Reporter(first_name='John', last_name='Smith', email='john@example.com')\n>>> r.save()\n\n>>> r2 = Reporter(first_name='Paul', last_name='Jones', email='paul@example.com')\n>>> r2.save()\n
\n

创建一个 Article：

\n
Python console
>>> from datetime import date\n>>> a = Article(id=None, headline="This is a test", pub_date=date(2005, 7, 27), reporter=r)\n>>> a.save()\n\n>>> a.reporter.id\n1\n\n>>> a.reporter\n<Reporter: John Smith>\n
\n

注意你必须先保存对象，然后再给它指定外键关系。比如，使用未保存的 Reporter 创建 Article ，会引发 ValueError：

\n
Python console
>>> r3 = Reporter(first_name='John', last_name='Smith', email='john@example.com')\n>>> Article.objects.create(headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3)\nTraceback (most recent call last):\n...\nValueError: save() prohibited to prevent data loss due to unsaved related object 'reporter'.\n
\n

Article 对象可以访问与它们相关联的 Reporter 对象：

\n
Python console
>>> r = a.reporter\n
\n

通过 Reporter 对象来创建一个 Article

\n
Python console
>>> new_article = r.article_set.create(headline="John's second story", pub_date=date(2005, 7, 29))\n>>> new_article\n<Article: John's second story>\n>>> new_article.reporter\n<Reporter: John Smith>\n>>> new_article.reporter.id\n1\n
\n

创建一个新的article：

\n
Python console
>>> new_article2 = Article.objects.create(headline="Paul's story", pub_date=date(2006, 1, 17), reporter=r)\n>>> new_article2.reporter\n<Reporter: John Smith>\n>>> new_article2.reporter.id\n1\n>>> r.article_set.all()\n<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>\n
\n

添加相同的article到一个不同的article集合，请观察它怎么移动：

\n
Python console
>>> r2.article_set.add(new_article2)\n>>> new_article2.reporter.id\n2\n>>> new_article2.reporter\n<Reporter: Paul Jones>\n
\n

添加错误类型的对象，会引发TypeError：

\n
Python console
>>> r.article_set.add(r2)\nTraceback (most recent call last):\n...\nTypeError: 'Article' instance expected, got <Reporter: Paul Jones>\n\n>>> r.article_set.all()\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n>>> r2.article_set.all()\n<QuerySet [<Article: Paul's story>]>\n\n>>> r.article_set.count()\n2\n\n>>> r2.article_set.count()\n1\n
\n

注意在最后的例子里，那篇 article 的 reporter 已经从 John 变为 Paul。

\n

相关管理器也提供字段查询。只要你需要，API会自动跟踪关系，使用双下划线来分隔，你可以根据需要获取深层关系。这没有限制。比如：

\n
Python console
>>> r.article_set.filter(headline__startswith='This')\n<QuerySet [<Article: This is a test>]>\n\n# Find all Articles for any Reporter whose first name is "John".\n>>> Article.objects.filter(reporter__first_name='John')\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
\n

这里的查询是完全匹配：

\n
Python console
>>> Article.objects.filter(reporter__first_name='John')\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
\n

在相关字段上查询两次。这里转化成WHERE从句里的AND条件。

\n
Python console
>>> Article.objects.filter(reporter__first_name='John', reporter__last_name='Smith')\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
\n

对于相关查询，你可以提供主键值或显式传递相关对象：

\n
Python console
>>> Article.objects.filter(reporter__pk=1)\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n>>> Article.objects.filter(reporter=1)\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n>>> Article.objects.filter(reporter=r)\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n\n>>> Article.objects.filter(reporter__in=[1,2]).distinct()\n<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>\n>>> Article.objects.filter(reporter__in=[r,r2]).distinct()\n<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>\n
\n

你也可以使用查询集而不是实例的文字列表：

\n
Python console
>>> Article.objects.filter(reporter__in=Reporter.objects.filter(first_name='John')).distinct()\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
\n

反向查询：

\n
Python console
>>> Reporter.objects.filter(article__pk=1)\n<QuerySet [<Reporter: John Smith>]>\n>>> Reporter.objects.filter(article=1)\n<QuerySet [<Reporter: John Smith>]>\n>>> Reporter.objects.filter(article=a)\n<QuerySet [<Reporter: John Smith>]>\n\n>>> Reporter.objects.filter(article__headline__startswith='This')\n<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>\n>>> Reporter.objects.filter(article__headline__startswith='This').distinct()\n<QuerySet [<Reporter: John Smith>]>\n
\n

反向查询的计数与 distinct() ：

\n
Python console
>>> Reporter.objects.filter(article__headline__startswith='This').count()\n3\n>>> Reporter.objects.filter(article__headline__startswith='This').distinct().count()\n1\n
\n

可以循环查询：

\n
Python console
>>> Reporter.objects.filter(article__reporter__first_name__startswith='John')\n<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>\n>>> Reporter.objects.filter(article__reporter__first_name__startswith='John').distinct()\n<QuerySet [<Reporter: John Smith>]>\n>>> Reporter.objects.filter(article__reporter=r).distinct()\n<QuerySet [<Reporter: John Smith>]>\n
\n

如果删除了一个reporter，他的articles将被删除（假设使用设置了 CASCADE 的 django.db.models.ForeignKey.on_delete 来定义主键，这是默认设置）：

\n
Python console
>>> Article.objects.all()\n<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>\n>>> Reporter.objects.order_by('first_name')\n<QuerySet [<Reporter: John Smith>, <Reporter: Paul Jones>]>\n>>> r2.delete()\n>>> Article.objects.all()\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n>>> Reporter.objects.order_by('first_name')\n<QuerySet [<Reporter: John Smith>]>\n
\n

可以在查询中使用JOIN进行删除：

\n
Python console
>>> Reporter.objects.filter(article__headline__startswith='This').delete()\n>>> Reporter.objects.all()\n<QuerySet []>\n>>> Article.objects.all()\n<QuerySet []>\n
","rootId":"many-to-one-relationships","toc":[],"breadcrumbs":[{"docname":"topics/index","title":"使用 Django","url":"/zh-hans/3.0/topics/"},{"docname":"topics/db/index","title":"模型和数据库","url":"/zh-hans/3.0/topics/db/"},{"docname":"topics/db/examples/index","title":"模型关联 API 用法示例","url":"/zh-hans/3.0/topics/db/examples/"}],"prev":{"docname":"topics/db/examples/many_to_many","title":"多对多关联","url":"/zh-hans/3.0/topics/db/examples/many_to_many/"},"next":{"docname":"topics/db/examples/one_to_one","title":"一对一关联","url":"/zh-hans/3.0/topics/db/examples/one_to_one/"},"formats":{"html":"/zh-hans/3.0/topics/db/examples/many_to_one/","markdown":"/zh-hans/3.0/topics/db/examples/many_to_one.md","json":"/zh-hans/3.0/topics/db/examples/many_to_one.json"},"source":"https://github.com/django/django/blob/stable/3.0.x/docs/topics/db/examples/many_to_one.txt","official":"https://docs.djangoproject.com/zh-hans/3.0/topics/db/examples/many_to_one/","inVersions":["6.1","6.0","5.2","5.1","5.0","4.2","4.1","4.0","3.2","3.1","3.0","2.2","2.1","2.0"],"inLocales":["en","zh-hans","fr","ja","id","pt-br","ko","es","el","pl"]}