多对一关联关系

使用ForeignKey 定义多对一关联关系：

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):              # __unicode__ on Python 2
        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)

    def __str__(self):              # __unicode__ on Python 2
        return self.headline

    class Meta:
        ordering = ('headline',)

接下来是可以使用Python API 执行的操作的示例。

创建几个Reporter：

>>> r = Reporter(first_name='John', last_name='Smith', email='[email protected]')
>>> r.save()

>>> r2 = Reporter(first_name='Paul', last_name='Jones', email='[email protected]')
>>> 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，将对象分配给一个外键之前必须保存。例如，使用未保存的Reporter 创建Article 将引发ValueError：

>>> r3 = Reporter(first_name='John', last_name='Smith', email='[email protected]')
>>> Article(headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3)
Traceback (most recent call last):
...
ValueError: 'Cannot assign "<Reporter: John Smith>": "Reporter" instance isn't saved in the database.'

如果想关闭对未保存对象的检查，可以使用allow_unsaved_instance_assignment 属性。

Django 1.8 中的更改：

以前，赋值尚未保存的对象不会引发错误，这可能会导致数据丢失而毫无提示。

Article 对象可以访问与其关联的Reporter 对象︰

>>> r = a.reporter

在Python 2中，这些是类型str的字符串，而不是unicode字符串，因为这是在创建这个报告器时使用的（我们没有刷新数据库中的数据，它总是返回unicode字符串）：

>>> r.first_name, r.last_name
('John', 'Smith')

通过Reporter对象创建一个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

创建一篇新的Article，并将它添加到Article 集︰

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

将同一篇文章添加到不同的文章集 - 检查它移动：

>>> 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

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

>>> r.article_set.count()
2

>>> r2.article_set.count()
1

注意，在最后一个例子中，文章已经从John移动到Paul。

相关管理器也支持字段查找。API会根据您的需要自动遵循关系。使用双下划线分隔关系。这个工作级别很深，你想要的。Related managers support field lookups as well. The API automatically follows relationships as far as you need. Use double underscores to separate relationships.例如：

>>> r.article_set.filter(headline__startswith='This')
[<Article: This is a test>]

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

这里隐含精确匹配︰

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

对关联的字段进行两次查询。这将转换为WHERE 子句中的AND 条件︰

>>> Article.objects.filter(reporter__first_name='John', reporter__last_name='Smith')
[<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)
[<Article: John's second story>, <Article: This is a test>]
>>> Article.objects.filter(reporter=1)
[<Article: John's second story>, <Article: This is a test>]
>>> Article.objects.filter(reporter=r)
[<Article: John's second story>, <Article: This is a test>]

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

反向查询：

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

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

在相反方向计数与distinct()结合使用：

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

查询可以圆圈：

>>> Reporter.objects.filter(article__reporter__first_name__startswith='John')
[<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]
>>> Reporter.objects.filter(article__reporter__first_name__startswith='John').distinct()
[<Reporter: John Smith>]
>>> Reporter.objects.filter(article__reporter=r).distinct()
[<Reporter: John Smith>]

如果删除作者，他的文章将被删除（假设ForeignKey是使用django.db.models.ForeignKey.on_delete设置为CASCADE定义的， ）：

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

你可以在查询中直接使用删除操作︰

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