多对一关联

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：

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：

>>> 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：

>>> 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：

>>> 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：

>>> r = a.reporter

通过 Reporter 对象创建一篇 Article：

>>> 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：

>>> 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：

>>> 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：

>>> 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：

>>> 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：

>>> 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：

>>> 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>]>

在相反的方向进行查询：

>>> 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>]>

在相反的方向进行计数与 distinct() 结合使用：

>>> 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：

>>> 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 设置为 CASCADE 来定义外键，这是默认值）：

>>> 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：

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