{"title":"多对一关联","version":"2.2","locale":"zh-hans","docname":"topics/db/examples/many_to_one","url":"/zh-hans/2.2/topics/db/examples/many_to_one/","canonical":"https://djangodocs.dev/zh-hans/2.2/topics/db/examples/many_to_one/","summary":"要定义一个多对一关联，使用 ForeignKey : Code Copy from django.db import models class Reporter ( models . Model ): first_name = models . CharField ( max_length = 30 ) last_name =…","html":"

多对一关联Link to this heading#

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要定义一个多对一关联，使用 ForeignKey:

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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
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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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创建一些 Reporters：

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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
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创建一个 Article：

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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
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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：

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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
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Article 对象可以访问与它们相关联的 Reporter 对象：

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Python console
>>> r = a.reporter\n
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通过 Reporter 对象来创建一个 Article

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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
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创建一个新的article：

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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
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添加相同的article到一个不同的article集合，请观察它怎么移动：

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Python console
>>> r2.article_set.add(new_article2)\n>>> new_article2.reporter.id\n2\n>>> new_article2.reporter\n<Reporter: Paul Jones>\n
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添加错误类型的对象，会引发TypeError：

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

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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：

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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
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这里的查询是完全匹配：

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Python console
>>> Article.objects.filter(reporter__first_name='John')\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
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在相关字段上查询两次。这里转化成WHERE从句里的AND条件。

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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
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对于相关查询，你可以提供主键值或显式传递相关对象：

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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
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你也可以使用查询集而不是实例的文字列表：

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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
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反向查询：

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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
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反向查询的计数与 distinct() ：

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Python console
>>> Reporter.objects.filter(article__headline__startswith='This').count()\n3\n>>> Reporter.objects.filter(article__headline__startswith='This').distinct().count()\n1\n
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可以循环查询：

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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
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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）：

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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
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可以在查询中使用JOIN进行删除：

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Python console
>>> Reporter.objects.filter(article__headline__startswith='This').delete()\n>>> Reporter.objects.all()\n<QuerySet []>\n>>> Article.objects.all()\n<QuerySet []>\n
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