使用 ForeignKey 來定義一個多對一的關係。

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',)

以下是可以使用Python API工具執行的操作示例。

建立一些記者：

>>> 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>

請注意，必須先儲存物件，然後才能將其分配給外來鍵關係。例如，Article使用未儲存的Reporter 來建立，將會報錯ValueError：

>>> r3 = Reporter(first_name='John', last_name='Smith', email='[email protected]')
>>> 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'.

文章物件可以訪問與其相關的Reporter物件：

>>> r = a.reporter

通過Reporter物件建立文章：

>>> 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

建立一篇新文章：

>>> 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>]>

將相同的文章新增到不同的文章集 - 檢查它是否移動：

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

新增錯誤型別的物件會引發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

請注意，在最後一個示例中，文章已從John轉移到Paul。

關聯的 managers 也支援欄位查詢。API會根據您的需要自動跟蹤關係。使用雙下劃線來分隔關係。這可以根據需要進行多級操作。沒有限制。例如：

>>> 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>]>

這裡完全匹配：

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

在相關欄位上查詢兩次。這轉換為用AND連線WHERE子句中的條件：

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

對於相關查詢，您可以提供主鍵值或顯式傳遞相關物件：

>>> 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>]>

您還可以使用查詢集而不是文字例項列表：

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

向另一邊查詢：

>>> 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>]>

相反方向的計數與distinct()一起使用：

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

查詢可以圍成一圈：

>>> 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>]>

如果您刪除了一個記者，他的文章將被刪除（假設ForeignKey的on_delete屬性設定為 CASCADE，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>]>

您可以在查詢中使用刪除：

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