Django 一對多關系（Foreign Key）

我們使用Foreign Key 在model中來實作這種一對多關系。

from django.db import models

# 記者
class Reporter(models.Model):
	# 名字
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)
    # email資訊
    email = models.EmailField()
	# 重寫了print出來的資訊
    def __str__(self):
        return "%s %s" % (self.first_name, self.last_name)

class Article(models.Model):
	# 一篇文章，裡面包含标題，釋出時間，釋出者。
    headline = models.CharField(max_length=100)
    pub_date = models.DateField()
    reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
	# 重寫了print出來的資訊
    def __str__(self):
        return self.headline
	# 排序資訊：通過标題來排序
    class Meta:
        ordering = ('headline',)
           

建立記者

>>> 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()
           

建立文章

>>> 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>
           

你必須儲存了記者後才能建立文章。

文章和記者得到了關聯。

>>> r = a.reporter
           

通過記者對象去建立文章

>>> 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
           

建立一篇文章

>>> 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>]>

>>> r2.article_set.add(new_article2)
>>> new_article2.reporter.id
2
>>> new_article2.reporter
           

添加錯誤的類型會産生錯誤：

>>> 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
           

一系列操作後，文章從r1 移動到了r2 之後。

相關的管理器也支援字段查找。API會根據您的需要自動跟蹤關系。使用雙下劃線分隔關系。這可以工作到你想要的深度。是沒有限制的。例如:

過濾器查詢：

>>> 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>]>
           

and 查詢：

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

可以傳遞多種值

>>> 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>]>
           

您還可以使用queryset來代替執行個體的文本清單:

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

反方向查找:

>>> 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>]>
           

如果你想删除一個記者，那麼它的文章也會被聯級删除。

（假設外鍵是用django.db.models.ForeignKey定義的。on_delete設定為級聯，這是預設值):

>>> 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>]>
           

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