要定义一个多对一关联，使用 ​ForeignKey​:

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 "%s %s" % (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。

创建一些 ​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()

创建一个 ​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>

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​：

>>> 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 ​对象可以访问与它们相关联的 ​Reporter ​对象：

>>> r = a.reporter

通过 ​Reporter ​对象来创建一个 ​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

创建一个新的​article​：

>>> 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>]>

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

>>> 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：

>>> 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>]>

这里的查询是完全匹配：

>>> 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。这里转化成​WHERE​子句里的​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>]>

对于相关查询，你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>]>

你也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>]>

反向查询的计数与 ​distinct()​ ：

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

可以循环查询：

>>> 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​reporter​，他的​articlles​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）：

>>> 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>]>

可以在查询中使用JOIN进行删除：

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