Django4.0 模型關聯(lián)-多對多關聯(lián)

要定義多對多關系，請使用 ?ManyToManyField?。

在這個例子中，一個?Article?可以在多個 ?Publication ?對象中發(fā)布，而一個 ?Publication ?有多個 ?Article ?對象：

from django.db import models

class Publication(models.Model):
    title = models.CharField(max_length=30)

    class Meta:
        ordering = ['title']

    def __str__(self):
        return self.title

class Article(models.Model):
    headline = models.CharField(max_length=100)
    publications = models.ManyToManyField(Publication)

    class Meta:
        ordering = ['headline']

    def __str__(self):
        return self.headline

以下是可以使用 Python API 工具執(zhí)行的操作示例。

創(chuàng)建一些 ?Publications?:

>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> p2 = Publication(title='Science News')
>>> p2.save()
>>> p3 = Publication(title='Science Weekly')
>>> p3.save()

創(chuàng)建一個?Article?:

>>> a1 = Article(headline='Django lets you build web apps easily')

在保存之前，您不能將其與 ?Publications?關聯(lián)：

>>> a1.publications.add(p1)
Traceback (most recent call last):
...
ValueError: "<Article: Django lets you build web apps easily>" needs to have a value for field "id" before this many-to-many relationship can be used.

然后保存它

>>> a1.save()

用一個 ?Publication?來關聯(lián) ?Article?:

>>> a1.publications.add(p1)

創(chuàng)建另一個?Article?, 并且設置它的?Publications?

>>> a2 = Article(headline='NASA uses Python')
>>> a2.save()
>>> a2.publications.add(p1, p2)
>>> a2.publications.add(p3)

二次添加是可以的，但它不會重復這一關系。

>>> a2.publications.add(p3)

添加錯誤類型的對象會引發(fā)：exc：TypeError

>>> a2.publications.add(a1)
Traceback (most recent call last):
...
TypeError: 'Publication' instance expected

使用 create() 一步創(chuàng)建 ?Publications?并將其添加到 ?Article?：

>>> new_publication = a2.publications.create(title='Highlights for Children')

?Article ?對象可以訪問與它們相關的 ?Publication ?對象：

>>> a1.publications.all()
<QuerySet [<Publication: The Python Journal>]>
>>> a2.publications.all()
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>

?Publication ?對象可以訪問與它們相關的 ?Article ?對象：

>>> p2.article_set.all()
<QuerySet [<Article: NASA uses Python>]>
>>> p1.article_set.all()
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Publication.objects.get(id=4).article_set.all()
<QuerySet [<Article: NASA uses Python>]>

可通過 跨關聯(lián)查詢 查詢多對多關聯(lián):

>>> Article.objects.filter(publications__id=1)
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Article.objects.filter(publications__pk=1)
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Article.objects.filter(publications=1)
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Article.objects.filter(publications=p1)
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>

>>> Article.objects.filter(publications__title__startswith="Science")
<QuerySet [<Article: NASA uses Python>, <Article: NASA uses Python>]>

>>> Article.objects.filter(publications__title__startswith="Science").distinct()
<QuerySet [<Article: NASA uses Python>]>

?count()? 函數(shù)也可以配合 ?distinct()? 

>>> Article.objects.filter(publications__title__startswith="Science").count()
2

>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()
1

>>> Article.objects.filter(publications__in=[1,2]).distinct()
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Article.objects.filter(publications__in=[p1,p2]).distinct()
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>

支持反向m2m查詢（比如，從沒有 ?ManyToManyField ?的表開始）：

>>> Publication.objects.filter(id=1)
<QuerySet [<Publication: The Python Journal>]>
>>> Publication.objects.filter(pk=1)
<QuerySet [<Publication: The Python Journal>]>

>>> Publication.objects.filter(article__headline__startswith="NASA")
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>

>>> Publication.objects.filter(article__id=1)
<QuerySet [<Publication: The Python Journal>]>
>>> Publication.objects.filter(article__pk=1)
<QuerySet [<Publication: The Python Journal>]>
>>> Publication.objects.filter(article=1)
<QuerySet [<Publication: The Python Journal>]>
>>> Publication.objects.filter(article=a1)
<QuerySet [<Publication: The Python Journal>]>

>>> Publication.objects.filter(article__in=[1,2]).distinct()
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
>>> Publication.objects.filter(article__in=[a1,a2]).distinct()
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>

排除相關條目的工作方式正如你所期望的那樣（盡管相關 SQL 有點復雜）：

>>> Article.objects.exclude(publications=p2)
<QuerySet [<Article: Django lets you build web apps easily>]>

如果我們刪除 ?Publication?， 它的 ?Articles ?無法訪問它：

>>> p1.delete()
>>> Publication.objects.all()
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]>
>>> a1 = Article.objects.get(pk=1)
>>> a1.publications.all()
<QuerySet []>

如果我們刪除了 ?Article?，它的 ?Publications ?也無法訪問它：

>>> a2.delete()
>>> Article.objects.all()
<QuerySet [<Article: Django lets you build web apps easily>]>
>>> p2.article_set.all()
<QuerySet []>

通過m2m的另一端添加：

>>> a4 = Article(headline='NASA finds intelligent life on Earth')
>>> a4.save()
>>> p2.article_set.add(a4)
>>> p2.article_set.all()
<QuerySet [<Article: NASA finds intelligent life on Earth>]>
>>> a4.publications.all()
<QuerySet [<Publication: Science News>]>

使用關鍵字通過另一端添加：

>>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')
>>> p2.article_set.all()
<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
>>> a5 = p2.article_set.all()[1]
>>> a5.publications.all()
<QuerySet [<Publication: Science News>]>

從 ?Article ?中移除 ?Publication?：

>>> a4.publications.remove(p2)
>>> p2.article_set.all()
<QuerySet [<Article: Oxygen-free diet works wonders>]>
>>> a4.publications.all()
<QuerySet []>

從另一端移除：

>>> p2.article_set.remove(a5)
>>> p2.article_set.all()
<QuerySet []>
>>> a5.publications.all()
<QuerySet []>

可以設置關系：

>>> a4.publications.all()
<QuerySet [<Publication: Science News>]>
>>> a4.publications.set([p3])
>>> a4.publications.all()
<QuerySet [<Publication: Science Weekly>]>

關系可以被清除：

>>> p2.article_set.clear()
>>> p2.article_set.all()
<QuerySet []>

可以從另一端清除：

>>> p2.article_set.add(a4, a5)
>>> p2.article_set.all()
<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
>>> a4.publications.all()
<QuerySet [<Publication: Science News>, <Publication: Science Weekly>]>
>>> a4.publications.clear()
>>> a4.publications.all()
<QuerySet []>
>>> p2.article_set.all()
<QuerySet [<Article: Oxygen-free diet works wonders>]>

重新創(chuàng)建我們剛刪除的 ?Article ?和 ?Publication ?：

>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> a2 = Article(headline='NASA uses Python')
>>> a2.save()
>>> a2.publications.add(p1, p2, p3)

批量刪除一些?publications ?- 對已刪除?publications?的引用應該去：

>>> Publication.objects.filter(title__startswith='Science').delete()
>>> Publication.objects.all()
<QuerySet [<Publication: Highlights for Children>, <Publication: The Python Journal>]>
>>> Article.objects.all()
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA finds intelligent life on Earth>, <Article: NASA uses Python>, <Article: Oxygen-free diet works wonders>]>
>>> a2.publications.all()
<QuerySet [<Publication: The Python Journal>]>

批量刪除?Article?：

>>> q = Article.objects.filter(headline__startswith='Django')
>>> print(q)
<QuerySet [<Article: Django lets you build web apps easily>]>
>>> q.delete()

在 ?delete()? 之后，需要清除 ?QuerySet ?緩存，并且引用的對象應該沒有了：

>>> print(q)
<QuerySet []>
>>> p1.article_set.all()
<QuerySet [<Article: NASA uses Python>]>