多对一关系

要定义多对一关系，请使用 ForeignKey 。

在此示例中，一个 Reporter 可以与多个 Article 对象，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。

创造几个记者：

>>> 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()

创建一篇文章：

>>> 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，必须先保存对象，然后才能将其分配给外键关系。例如，创建一个 Article 未保存 Reporter 加薪 ValueError ：

>>> 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'.

文章对象可以访问其相关的Reporter对象：

>>> r = a.reporter

通过Reporter对象创建一篇文章：

>>> 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

创建一篇新文章：

>>> 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>]>

将同一文章添加到不同的文章集中-检查它是否移动：

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

添加类型错误的对象会引发TypeError：

>>> 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，在上一个例子中，文章已经从约翰移到了保罗。

相关管理器也支持字段查找。API会根据您的需要自动跟踪关系。使用双下划线分隔关系。这可以根据您的需要进行任意多个级别的操作。没有限制。例如：

>>> 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>]>

此处隐含的是完全匹配：

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

在相关字段上查询两次。这将转换为WHERE子句中的AND条件：

>>> 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：

>>> 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：

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

反向查询：

>>> 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>]>

以相反的方向计数与 distinct() ：

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

查询可以循环进行：

>>> 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>]>

如果删除记者，他们的文章也会被删除(假设ForeignKey是用 django.db.models.ForeignKey.on_delete 设置为 CASCADE ，这是默认设置)：

>>> 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联接删除：

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