django之多对多关系

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2024-10-31 21:08:39 212人阅读

django 1.7

定义多对多关系，使用 ManyToManyFiled。

本例中，文章Article可以在多个出版物Publication上发表，而出版物可以有多个文章对象。

from django.db import models

class Publication(models.Model):
    title = models.CharField(max_length=30)
    
    def __str__(self):
        return self.title
    
    class Meta:
        ordering = ('title',)


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

    def __str__(self):
        return self.headline

    class Meta:
        ordering = ('headline',)

下面是使用Python API执行例子的操作。注意，若你为多对多关系使用了中间体模型，有些关系管理器的方法不能使用，因此这些例子中某些在这种模型下不能工作。

创建几个出版物：

>>> p1 = Publication(title=‘The Python Journal‘)

>>> p1.save()

>>> p2 = Publication(title=‘Science News‘)

>>> p2.save()

>>> p3 = Publication(title=‘Science wekkly‘)

创建文章：

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

在保存它以前，不能与一个Publication关联。

>>> a1.publications.add(p1)

...

ValueError: ‘Article‘ instance needs to have a primary key value before a many-to-many relationship can be used.

保存

>>> a1.save()

把Article关联到Publication:

>>> a1.publications.add(p1)

创建另一个Article，设定它在两个Publications中出现：

>>> a2 = Article(headline=‘NASA uses Python‘)

>>> a2.save()

>>> a2.publications.add(p1, p2)

>>> a2.publications.add(p3)

第2次添加也是正确的：

>>> a2.publications.add(p3)

添加一个错误类型的对象会产生一个 TypeError：

>>> a2.publictaions.add(a1)

Traceback (most recent call last):

...

TypeError: ‘Publication‘ instance excepted

使用create()一步创建Publications并把它添加到一个文章中：

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

Article对象可以访问它们关联的Publication对象：

>>> a1.publications.all()

[<Publication: HIghlights for Children>, ...]

>>> a2.publications.all()

[<Publication: Highlights for Children>, <Publication: Science News>, ... ]

Publication 对象可以访问它们关联的Article对象：

>>> p2.article_set.all()

[<Article: NASA uses Python>]

>>> p1.article_set.all()

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Publication.objects.get(id=4).article_set.all()

[<Article: NASA uses Python>]

可以使用跨关系查询来获得多对多关系查询：

>>> Article.objects.filter(publications__id=1)

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications__pk=1)

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications=1):

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications=p1)

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications__title__startwith=‘Science‘)

[<Article: NASA uses Python>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications__title__startwith=‘Science‘).distinct()

[<Article: NASA uses Python>]

count()函数也使用distinct()：

>>> Article.objects.filter(publications__title__startwith=‘Science‘).count()

>>> Article.objects.filter(publications__title__startwith=‘Science‘).distinct().count()

>>> Article.objects.filter(publications__in=[1,2]).distinct()

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

>>> Article.objects.filter(publications__in=[p1,p2]).distinct()

[<Article: Django lets you build Web app easyily>, <Article: NASA uses Python>]

反向m2m查询也被支持（如，从不含ManyToManyField字段的表开始）：

>>> Publication.objects.filter(id=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(pk=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(article__headline__startwith=‘NASA‘)

[<Publication: Highlights for Children>, <Publication: Science News>, ... ]

>>> Publication.objects.filter(article__id=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(article__pk=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(article=1)

[<Publication: The Python Journal>]

>>> Publication.objects.filter(article=a1)

[<Publication: The Python Journal>]

Publication.objects.filter(article__in=[1,2]).distinct()

[<Publication: Highlights for Children>, <Publication: Science News>, ... ]

>>> Publication.objects.filter(article__in=[a1,a2]).distinct()

[<Publication: Highlights for Children>, <Publication: Science News>, ... ]

排除一个关联项目会按预期工作（尽管使用的SQL有一点复杂）：

>>> Article.objects.exclude(publications=p2)

[<Article: Django lets you build Web apps easily>]

如果我们删除Publication，它的Article将不能访问它：

>>> p1.delete()

>>> Publication.objects.all()

[<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>‘]

>>> a1 = Article.objects.get(pk=1)

>>> a1.publications.all()

[ ]

如果我们删除一个Article，它的Publications将不能访问它：

>>> a2.delete()

>>>Article.objects.all()

[<Artilce: Django lets you bulid Web apps easily>]

>>> p2.article_set.all()

[ ]

通过m2m的另一端进行添加：

>>> a4 = Article(headline=‘NASA finds intelligent lif on Earth‘)

>>>a4.save()

>>>p2.article_set.add(a4)

>>> p2.article_set.all()

[<Article: NASA finds intelligent lif on Earth>]

>>> a4.publications.all()

[<Publication: Science News>]

通过另一端使用关键字添加：

>>> new_article = p2.article_set.create(headline=‘Oxygen-free diet works wonders‘)

>>> p2.article_set.all()

[<Article: NASA finds intelligent lif on Earth>, <Article: Oxygen-free diet works wonders> ]

>>> a5 = p2.article_set.all()[1]

>>> a5.publications.all()

[<Publication: Science News> ]

从1个Article移除Publication：

>>> a4.publications.remove(p2)

>>> p2.article_set.all()

[<Article: Oxygen-free diet works wonders>]

>>> a4.publications.all()

[ ]

而从另一端：

>>> p2.article_set.remove(a5)

>>>p2.article_set.all()

[ ]

>>> a5.publications.all()

[ ]

可以赋值关系集。赋值操作清除任何存在的集合成员：

>>> a4.publications.all()

[<Publication: Science News>]

>>> a4.publications = [p3]

>>> a4.publications.all()

[<Publication: Science Weekly>]

关系集可以清除：

>>> p2.article_set.clear()

>>> p2.article_set.all()

[ ]

也可以从另一端做清除：

>>> p2.article_set.add(a4, a5)

>>> p2.article_set.all()

[<Article: NASA finds intelligent lif on Earth>, <Article: Oxygen-free diet works wonders> ]

>>> a4.publications.clear()

>>> a4.publications.all()

[ ]

>>> p2.article_set.all()

[<Article: Oxygen-free diet works wonders> ]

重新创建我们删除的Article和Publications:

>>> p1 = Publication(title=‘The Python Journal‘)

>>> p1.save()

>>> a2 = Article(headline=‘NASA uses Python‘)

>>> a2.save()

>>> a2.publications.add(p1, p2, p3)

批量删除一些Publications - 引用删除的publications也被删除：

>>> Publication.objects.filter(title__startwith=‘Science‘).delete()

>>> Publication.objects.all()

[<Publication: Highlights for Children>, <Publication: The Python Journal>]

>>> Article.objects.all()

[<Article: Django lets you build Web apps easily>, .... ]

>>> a2.publications.all()

[<Publication: The Python Journal>]

批量删除一些article - 引用删除的对象也被删除：

>>> q = Article.objects.filter(headline__startwith=‘Django‘)

>>> print(q)

[<Article: Django lets you build Web apps easily>]

>>> q.delete()

在delete()调用后，QeruySet缓存需要清除，而且音乐的对象也会删掉：

>>> print(q)

[ ]

>>> p1.article_set.all()

[<Article: NASA uses Python>]

调用clear()的替代方法是赋值空集：

>>> p1.article_set = []

>>> p1.article_set.all()

>>> a2.publications = [p1, new_publication]

>>> a2.publications.all()

[<Publication: Highlights for Children>, <Publication: The Python Journal>]

>>> a2.publications = [ ]

>>> a2.publications.all()

[ ]

django之多对多关系

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django之多对多关系

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