【Django】 ManyToManyField的操作方法

在这个示例中，可以在多个发布对象中发布文章，并且发布具有多个Article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',)

下面是可以使用Python API设施执行的操作示例。请注意，如果您使用的是一个多对多关系的中间模型，那么一些相关的管理器的方法是禁用的，因此这些例子中的一些将无法使用这些模型。

 

 

创建一些出版物:

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

创建一篇文章:

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

你不能把它与出版物联系在一起，直到它被保存下来:

>>> a1.publications.add(p1)
Traceback (most recent call last):
...
ValueError: 'Article' instance needs to have a primary key value before a many-to-many relationship can be used.

先保存：

>>> a1.save()

将这篇文章与出版物联系起来:

>>> a1.publications.add(p1)

创建另一篇文章，并将其设置为在两种出版物中出现:

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

添加错误类型的对象会引发类型错误:

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

使用Create()创建并添加一个发布到文章中:

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

文章对象可以访问其相关的发布对象:

>>> 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>]>

发布对象可以访问它们的相关文章对象:

>>> 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>]>

多对多关系可以在关系中使用查找来查询:

>>> 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>]>

删除一篇文章的出版物:

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

删除出版物的一篇文章:

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

可设置关系集:

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

关系集可以被清除:

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

详情请看:

https://docs.djangoproject.com/en/2.0/topics/db/examples/many_to_many/