---
title: "多对一关联"
version: 6.1
locale: zh-hans
source: https://docs.djangoproject.com/zh-hans/6.1/topics/db/examples/many_to_one/
canonical: https://djangodocs.dev/zh-hans/6.1/topics/db/examples/many_to_one/
---
# 多对一关联

要定义多对一关系，请使用 [`ForeignKey`](/zh-hans/6.1/ref/models/fields/#django.db.models.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：

```
from django.db import models

class Reporter(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)
    email = models.EmailField()

    def __str__(self):
        return f"{self.first_name} {self.last_name}"

class Article(models.Model):
    headline = models.CharField(max_length=100)
    pub_date = models.DateField()
    reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)

    def __str__(self):
        return self.headline

    class Meta:
        ordering = ["headline"]
```

下面是可以使用PythonAPI工具执行的操作示例。

创建几个 `Reporters`：

```pycon
>>> r = Reporter(first_name="John", last_name="Smith", email="john@example.com")
>>> r.save()

>>> r2 = Reporter(first_name="Paul", last_name="Jones", email="paul@example.com")
>>> r2.save()
```

创建一篇 `Article`：

```pycon
>>> from datetime import date
>>> a = Article(id=None, headline="This is a test", pub_date=date(2005, 7, 27), reporter=r)
>>> a.save()

>>> a.reporter.id
1

>>> a.reporter
<Reporter: John Smith>
```

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`：

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

`Article` 对象可以访问与其相关联的 `Reporter` 对象：

```pycon
>>> r = a.reporter
```

通过 `Reporter` 对象创建一篇 `Article`：

```pycon
>>> new_article = r.article_set.create(
...     headline="John's second story", pub_date=date(2005, 7, 29)
... )
>>> new_article
<Article: John's second story>
>>> new_article.reporter
<Reporter: John Smith>
>>> new_article.reporter.id
1
```

创建一篇新的文章：

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

将相同的文章添加到不同的文章集合中 - 检查它是否移动：

```pycon
>>> r2.article_set.add(new_article2)
>>> new_article2.reporter.id
2
>>> new_article2.reporter
<Reporter: Paul Jones>
```

添加错误类型的对象会引发 TypeError：

```pycon
>>> r.article_set.add(r2)
Traceback (most recent call last):
...
TypeError: 'Article' instance expected, got <Reporter: Paul Jones>

>>> r.article_set.all()
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
>>> r2.article_set.all()
<QuerySet [<Article: Paul's story>]>

>>> r.article_set.count()
2

>>> r2.article_set.count()
1
```

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

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：

```pycon
>>> r.article_set.filter(headline__startswith="This")
<QuerySet [<Article: This is a test>]>

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

这里隐含了精确匹配：

```pycon
>>> Article.objects.filter(reporter__first_name="John")
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
```

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：

```pycon
>>> Article.objects.filter(reporter__first_name="John", reporter__last_name="Smith")
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
```

对于相关查找，您可以提供主键值或显式传递相关对象：

```pycon
>>> Article.objects.filter(reporter__pk=1)
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
>>> Article.objects.filter(reporter=1)
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
>>> Article.objects.filter(reporter=r)
<QuerySet [<Article: John's second story>, <Article: This is a test>]>

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

您还可以使用查询集而不是实例的文字列表：

```pycon
>>> Article.objects.filter(
...     reporter__in=Reporter.objects.filter(first_name="John")
... ).distinct()
<QuerySet [<Article: John's second story>, <Article: This is a test>]>
```

在相反的方向进行查询：

```pycon
>>> Reporter.objects.filter(article__pk=1)
<QuerySet [<Reporter: John Smith>]>
>>> Reporter.objects.filter(article=1)
<QuerySet [<Reporter: John Smith>]>
>>> Reporter.objects.filter(article=a)
<QuerySet [<Reporter: John Smith>]>

>>> Reporter.objects.filter(article__headline__startswith="This")
<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
>>> Reporter.objects.filter(article__headline__startswith="This").distinct()
<QuerySet [<Reporter: John Smith>]>
```

在相反的方向进行计数与 `distinct()` 结合使用：

```pycon
>>> Reporter.objects.filter(article__headline__startswith="This").count()
3
>>> Reporter.objects.filter(article__headline__startswith="This").distinct().count()
1
```

查询可以循环：

```pycon
>>> Reporter.objects.filter(article__reporter__first_name__startswith="John")
<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
>>> Reporter.objects.filter(article__reporter__first_name__startswith="John").distinct()
<QuerySet [<Reporter: John Smith>]>
>>> Reporter.objects.filter(article__reporter=r).distinct()
<QuerySet [<Reporter: John Smith>]>
```

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`](/zh-hans/6.1/ref/models/fields/#django.db.models.ForeignKey.on_delete) 设置为 `CASCADE` 来定义外键，这是默认值）：

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

您可以在查询中使用 JOIN 进行删除操作：

```pycon
>>> Reporter.objects.filter(article__headline__startswith="This").delete()
>>> Reporter.objects.all()
<QuerySet []>
>>> Article.objects.all()
<QuerySet []>
```