多對一關係¶
要定義多對一關係，請使用 ForeignKey。
在這個範例中，一個 Reporter 可以與多個 Article 物件相關聯，但一個 Article 只能有一個 Reporter 物件
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 f"{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"]
以下範例說明可以使用 Python API 功能執行的操作。
建立幾個 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()
建立一篇文章
>>> 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，您必須先儲存物件，才能將其指定給外鍵關係。 例如，使用未儲存的 Reporter 建立 Article 會引發 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 物件可以存取它們相關的 Reporter 物件
>>> r = a.reporter
透過 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>]>
將同一篇文章新增到不同的文章集 - 檢查它是否已移動
>>> r2.article_set.add(new_article2)
>>> new_article2.reporter.id
2
>>> new_article2.reporter
<Reporter: Paul Jones>
新增錯誤類型的物件會引發 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，在最後一個範例中，文章已從 John 移動到 Paul。
相關管理員也支援欄位查詢。 API 會自動追蹤您所需的關係。 使用雙底線來分隔關係。 這適用於您想要的深度層級。 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>]>
這裡暗示精確比對
>>> 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。 這會轉換為 WHERE 子句中的 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>]>
對於相關查詢，您可以提供主鍵值或明確傳遞相關物件
>>> 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>]>
您也可以使用 queryset 而不是實例的文字清單
>>> 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>]>
反向計數與 distinct() 結合使用
>>> Reporter.objects.filter(article__headline__startswith="This").count()
3
>>> Reporter.objects.filter(article__headline__startswith="This").distinct().count()
1
查詢可以繞圈
>>> 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>]>
如果您刪除一個記者，他們的文章將會被刪除（假設 ForeignKey 定義為 django.db.models.ForeignKey.on_delete 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>]>
您可以使用查詢中的 JOIN 刪除
>>> Reporter.objects.filter(article__headline__startswith="This").delete()
>>> Reporter.objects.all()
<QuerySet []>
>>> Article.objects.all()
<QuerySet []>
