多对一关联

要定义一个多对一关联,使用 ForeignKey:

  1. from django.db import models
  2. class Reporter(models.Model):
  3. first_name = models.CharField(max_length=30)
  4. last_name = models.CharField(max_length=30)
  5. email = models.EmailField()
  6. def __str__(self):
  7. return "%s %s" % (self.first_name, self.last_name)
  8. class Article(models.Model):
  9. headline = models.CharField(max_length=100)
  10. pub_date = models.DateField()
  11. reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
  12. def __str__(self):
  13. return self.headline
  14. class Meta:
  15. ordering = ['headline']

下面是可以使用PythonAPI工具执行的操作示例。

创建一些 Reporters:

  1. >>> r = Reporter(first_name='John', last_name='Smith', email='john@example.com')
  2. >>> r.save()
  3. >>> r2 = Reporter(first_name='Paul', last_name='Jones', email='paul@example.com')
  4. >>> r2.save()

创建一个 Article:

  1. >>> from datetime import date
  2. >>> a = Article(id=None, headline="This is a test", pub_date=date(2005, 7, 27), reporter=r)
  3. >>> a.save()
  4. >>> a.reporter.id
  5. 1
  6. >>> a.reporter
  7. <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

  1. >>> r3 = Reporter(first_name='John', last_name='Smith', email='john@example.com')
  2. >>> Article.objects.create(headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3)
  3. Traceback (most recent call last):
  4. ...
  5. 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:

  1. >>> r = a.reporter

通过 Reporter 对象来创建一个 Article

  1. >>> new_article = r.article_set.create(headline="John's second story", pub_date=date(2005, 7, 29))
  2. >>> new_article
  3. <Article: John's second story>
  4. >>> new_article.reporter
  5. <Reporter: John Smith>
  6. >>> new_article.reporter.id
  7. 1

创建一个新的article:

  1. >>> new_article2 = Article.objects.create(headline="Paul's story", pub_date=date(2006, 1, 17), reporter=r)
  2. >>> new_article2.reporter
  3. <Reporter: John Smith>
  4. >>> new_article2.reporter.id
  5. 1
  6. >>> r.article_set.all()
  7. <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:

  1. >>> r2.article_set.add(new_article2)
  2. >>> new_article2.reporter.id
  3. 2
  4. >>> new_article2.reporter
  5. <Reporter: Paul Jones>

添加错误类型的对象,会引发TypeError:

  1. >>> r.article_set.add(r2)
  2. Traceback (most recent call last):
  3. ...
  4. TypeError: 'Article' instance expected, got <Reporter: Paul Jones>
  5. >>> r.article_set.all()
  6. <QuerySet [<Article: John's second story>, <Article: This is a test>]>
  7. >>> r2.article_set.all()
  8. <QuerySet [<Article: Paul's story>]>
  9. >>> r.article_set.count()
  10. 2
  11. >>> r2.article_set.count()
  12. 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:

  1. >>> r.article_set.filter(headline__startswith='This')
  2. <QuerySet [<Article: This is a test>]>
  3. # Find all Articles for any Reporter whose first name is "John".
  4. >>> Article.objects.filter(reporter__first_name='John')
  5. <QuerySet [<Article: John's second story>, <Article: This is a test>]>

这里的查询是完全匹配:

  1. >>> Article.objects.filter(reporter__first_name='John')
  2. <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从句里的AND条件。

  1. >>> Article.objects.filter(reporter__first_name='John', reporter__last_name='Smith')
  2. <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:

  1. >>> Article.objects.filter(reporter__pk=1)
  2. <QuerySet [<Article: John's second story>, <Article: This is a test>]>
  3. >>> Article.objects.filter(reporter=1)
  4. <QuerySet [<Article: John's second story>, <Article: This is a test>]>
  5. >>> Article.objects.filter(reporter=r)
  6. <QuerySet [<Article: John's second story>, <Article: This is a test>]>
  7. >>> Article.objects.filter(reporter__in=[1,2]).distinct()
  8. <QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
  9. >>> Article.objects.filter(reporter__in=[r,r2]).distinct()
  10. <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:

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

反向查询:

  1. >>> Reporter.objects.filter(article__pk=1)
  2. <QuerySet [<Reporter: John Smith>]>
  3. >>> Reporter.objects.filter(article=1)
  4. <QuerySet [<Reporter: John Smith>]>
  5. >>> Reporter.objects.filter(article=a)
  6. <QuerySet [<Reporter: John Smith>]>
  7. >>> Reporter.objects.filter(article__headline__startswith='This')
  8. <QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
  9. >>> Reporter.objects.filter(article__headline__startswith='This').distinct()
  10. <QuerySet [<Reporter: John Smith>]>

反向查询的计数与 distinct() :

  1. >>> Reporter.objects.filter(article__headline__startswith='This').count()
  2. 3
  3. >>> Reporter.objects.filter(article__headline__startswith='This').distinct().count()
  4. 1

可以循环查询:

  1. >>> Reporter.objects.filter(article__reporter__first_name__startswith='John')
  2. <QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
  3. >>> Reporter.objects.filter(article__reporter__first_name__startswith='John').distinct()
  4. <QuerySet [<Reporter: John Smith>]>
  5. >>> Reporter.objects.filter(article__reporter=r).distinct()
  6. <QuerySet [<Reporter: John Smith>]>

如果删除了一个reporter,他的articlles文章将被删除(假设使用设置了 CASCADEdjango.db.models.ForeignKey.on_delete 来定义主键,这是默认设置):

  1. >>> Article.objects.all()
  2. <QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
  3. >>> Reporter.objects.order_by('first_name')
  4. <QuerySet [<Reporter: John Smith>, <Reporter: Paul Jones>]>
  5. >>> r2.delete()
  6. >>> Article.objects.all()
  7. <QuerySet [<Article: John's second story>, <Article: This is a test>]>
  8. >>> Reporter.objects.order_by('first_name')
  9. <QuerySet [<Reporter: John Smith>]>

可以在查询中使用JOIN进行删除:

  1. >>> Reporter.objects.filter(article__headline__startswith='This').delete()
  2. >>> Reporter.objects.all()
  3. <QuerySet []>
  4. >>> Article.objects.all()
  5. <QuerySet []>