To define a many-to-one relationship, use 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): # __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='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，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='john@example.com')
>>> 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。
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
On Python 2, these are strings of type str instead of unicode strings because that’s what was used in the creation of this reporter (and we haven’t refreshed the data from the database, which always returns unicode strings):
>>> 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>]
Add the same article to a different article set - check that it moves:
>>> 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
Note that in the last example the article has moved from John to Paul.
Related managers support field lookups as well. The API automatically follows relationships as far as you need. Use double underscores to separate relationships. This works as many levels deep as you want. Related managers support field lookups as well. The API automatically follows relationships as far as you need. Use double underscores to separate relationships.For example:
>>> 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>]
For the related lookup you can supply a primary key value or pass the related object explicitly:
>>> 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>]
You can also use a queryset instead of a literal list of instances:
>>> 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>]
Counting in the opposite direction works in conjunction with distinct():
>>> Reporter.objects.filter(article__headline__startswith='This').count()
3
>>> Reporter.objects.filter(article__headline__startswith='This').distinct().count()
1
Queries can go round in circles:
>>> 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()
[]
2015 年 5 月 13 日