Django4.0 模型關(guān)聯(lián)-多對一關(guān)聯(lián)

要定義一個多對一關(guān)聯(lián)，使用 ?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):
        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']

下面是可以使用PythonAPI工具執(zhí)行的操作示例。

創(chuàng)建一些 ?Reporters?：

>>> 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()

創(chuàng)建一個 ?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>

注意你必須先保存對象，然后再給它指定外鍵關(guān)系。比如，使用未保存的 ?Reporter ?創(chuàng)建 ?Article ?，會引發(fā) ?ValueError?：

>>> r3 = Reporter(first_name='John', last_name='Smith', email='john@example.com')
>>> 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'.

?Article ?對象可以訪問與它們相關(guān)聯(lián)的 ?Reporter ?對象：

>>> r = a.reporter

通過 ?Reporter ?對象來創(chuàng)建一個 ?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

創(chuàng)建一個新的?article?：

>>> 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>]>

添加相同的?article?到一個不同的?article?集合，請觀察它怎么移動：

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

添加錯誤類型的對象，會引發(fā)?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

注意在最后的例子里，那篇 ?article ?的 ?reporter ?已經(jīng)從 ?John ?變?yōu)??Paul?。

相關(guān)管理器也提供字段查詢。只要你需要，API會自動跟蹤關(guān)系，使用雙下劃線來分隔，你可以根據(jù)需要獲取深層關(guān)系。這沒有限制。比如：

>>> 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>]>

這里的查詢是完全匹配：

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

在相關(guān)字段上查詢兩次。這里轉(zhuǎn)化成?WHERE?子句里的?AND?條件。

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

對于相關(guān)查詢，你可以提供主鍵值或顯式傳遞相關(guān)對象：

>>> 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>]>

你也可以使用查詢集而不是實例的文字列表：

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

反向查詢：

>>> 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>]>

反向查詢的計數(shù)與 ?distinct()? ：

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

可以循環(huán)查詢：

>>> 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>]>

如果刪除了一個?reporter?，他的?articlles?將被刪除（假設(shè)使用設(shè)置了 ?CASCADE ?的 ?django.db.models.ForeignKey.on_delete? 來定義主鍵，這是默認(rèn)設(shè)置）：

>>> 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>]>

可以在查詢中使用JOIN進(jìn)行刪除：

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