{"title":"多对一关联","version":"5.2","locale":"zh-hans","docname":"topics/db/examples/many_to_one","url":"/zh-hans/5.2/topics/db/examples/many_to_one/","canonical":"https://djangodocs.dev/zh-hans/5.2/topics/db/examples/many_to_one/","summary":"要定义多对一关系，请使用 ForeignKey 。 在这个例子中，一个 Reporter 可以与多个 Article 对象关联，但一个 Article 只能有一个 Reporter 对象： Code Copy from django.db import models class Reporter ( models .…","html":"

多对一关联Link to this heading#

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要定义多对一关系，请使用 ForeignKey。

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在这个例子中，一个 Reporter 可以与多个 Article 对象关联，但一个 Article 只能有一个 Reporter 对象：

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Code
from django.db import models\n\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 f"{self.first_name} {self.last_name}"\n\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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创建几个 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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创建一篇 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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请注意，在将对象分配给外键关系之前，您必须保存该对象。例如，创建一个未保存的 Reporter 的 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(\n...     headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3\n... )\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 对象可以访问与其相关联的 Reporter 对象：

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Python console
>>> r = a.reporter\n
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通过 Reporter 对象创建一篇 Article：

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Python console
>>> new_article = r.article_set.create(\n...     headline="John's second story", pub_date=date(2005, 7, 29)\n... )\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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创建一篇新的文章：

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Python console
>>> new_article2 = Article.objects.create(\n...     headline="Paul's story", pub_date=date(2006, 1, 17), reporter=r\n... )\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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将相同的文章添加到不同的文章集合中 - 检查它是否移动：

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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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相关管理器也支持字段查找。API 会自动按需遍历关系。使用双下划线来分隔关系。这可以深入到您需要的任何级别，没有限制。例如：

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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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这里隐含了精确匹配：

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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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在相关字段上进行两次查询。这将在 WHERE 子句中转换为 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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对于相关查找，您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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您还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(\n...     reporter__in=Reporter.objects.filter(first_name="John")\n... ).distinct()\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
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在相反的方向进行查询：

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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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在相反的方向进行计数与 distinct() 结合使用：

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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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查询可以循环：

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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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如果删除了一个记者，他们的文章将被删除（假设使用 django.db.models.ForeignKey.on_delete 设置为 CASCADE 来定义外键，这是默认值）：

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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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您可以在查询中使用 JOIN 进行删除操作：

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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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