使用ManyToManyField
来定义多对多关系
在此示例中,可以在多个Article
对象中发布Publication
,并且Article
有多个Publication
from django.db import models
class Publication(models.Model):
title = models.CharField(max_length=30)
def __str__(self): # __unicode__ on Python 2
return self.title
class Meta:
ordering = ('title',)
class Article(models.Model):
headline = models.CharField(max_length=100)
publications = models.ManyToManyField(Publication)
def __str__(self): # __unicode__ on Python 2
return self.headline
class Meta:
ordering = ('headline',)
以下是可以使用Python API设施执行的操作的示例。 请注意,如果您对多对多关系使用an intermediate model,则会禁用某些相关管理器的方法,因此其中一些示例将不适用于此类模型。
创建几个Publications
:
>>> p1 = Publication(title='The Python Journal')
>>> p1.save()
>>> p2 = Publication(title='Science News')
>>> p2.save()
>>> p3 = Publication(title='Science Weekly')
>>> p3.save()
创建Article
:
>>> a1 = Article(headline='Django lets you build Web apps easily')
您无法将其与Publication
相关联,直到保存为止:
>>> 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.
保存!
>>> a1.save()
将Article
与Publication
:
>>> a1.publications.add(p1)
创建另一个Article
,并将其设置为在Publications
中显示:
>>> a2 = Article(headline='NASA uses Python')
>>> a2.save()
>>> a2.publications.add(p1, p2)
>>> a2.publications.add(p3)
添加2次也没关系:
>>> a2.publications.add(p3)
添加错误类型的对象会引起TypeError
:
>>> a2.publications.add(a1)
Traceback (most recent call last):
...
TypeError: 'Publication' instance expected
使用create()
在一个步骤中创建Publication
并添加到Article
:
>>> new_publication = a2.publications.create(title='Highlights for Children')
Article
对象可以访问其相关的Publication
对象:
>>> a1.publications.all()
]>
>>> a2.publications.all()
, , , ]>
Publication
对象可以访问其相关的Article
对象:
>>> p2.article_set.all()
]>
>>> p1.article_set.all()
, ]>
>>> Publication.objects.get(id=4).article_set.all()
]>
可以使用lookups across relationships查询多对多关系:
>>> Article.objects.filter(publications__id=1)
, ]>
>>> Article.objects.filter(publications__pk=1)
, ]>
>>> Article.objects.filter(publications=1)
, ]>
>>> Article.objects.filter(publications=p1)
, ]>
>>> Article.objects.filter(publications__title__startswith="Science")
, ]>
>>> Article.objects.filter(publications__title__startswith="Science").distinct()
]>
count()
函数也尊重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()
, ]>
>>> Article.objects.filter(publications__in=[p1,p2]).distinct()
, ]>
支持反向m2m查询(即,从不具有ManyToManyField
的表开始):
>>> Publication.objects.filter(id=1)
]>
>>> Publication.objects.filter(pk=1)
]>
>>> Publication.objects.filter(article__headline__startswith="NASA")
, , , ]>
>>> Publication.objects.filter(article__id=1)
]>
>>> Publication.objects.filter(article__pk=1)
]>
>>> Publication.objects.filter(article=1)
]>
>>> Publication.objects.filter(article=a1)
]>
>>> Publication.objects.filter(article__in=[1,2]).distinct()
, , , ]>
>>> Publication.objects.filter(article__in=[a1,a2]).distinct()
, , , ]>
排除相关项目的工作原理如你所期望的(虽然涉及的SQL有点复杂):
>>> Article.objects.exclude(publications=p2)
]>
如果我们删除Publication
,则其Articles
将无法访问:
>>> p1.delete()
>>> Publication.objects.all()
, , ]>
>>> a1 = Article.objects.get(pk=1)
>>> a1.publications.all()
如果我们删除Article
,则其Publications
将无法访问:
>>> a2.delete()
>>> Article.objects.all()
]>
>>> p2.article_set.all()
通过m2m的'other'端添加:
>>> a4 = Article(headline='NASA finds intelligent life on Earth')
>>> a4.save()
>>> p2.article_set.add(a4)
>>> p2.article_set.all()
]>
>>> a4.publications.all()
]>
使用关键字通过另一端添加:
>>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')
>>> p2.article_set.all()
, ]>
>>> a5 = p2.article_set.all()[1]
>>> a5.publications.all()
]>
从Publication
中删除Article
:
>>> a4.publications.remove(p2)
>>> p2.article_set.all()
]>
>>> a4.publications.all()
从另一端:
>>> p2.article_set.remove(a5)
>>> p2.article_set.all()
>>> a5.publications.all()
关系集可以设置:
>>> a4.publications.all()
]>
>>> a4.publications.set([p3])
>>> a4.publications.all()
]>
关系集可以清除:
>>> p2.article_set.clear()
>>> p2.article_set.all()
你可以从另一端清除:
>>> p2.article_set.add(a4, a5)
>>> p2.article_set.all()
, ]>
>>> a4.publications.all()
, ]>
>>> a4.publications.clear()
>>> a4.publications.all()
>>> p2.article_set.all()
]>
重新创建我们已删除的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)
批量删除一些Publications
- 对已删除出版物的引用应该是:
>>> Publication.objects.filter(title__startswith='Science').delete()
>>> Publication.objects.all()
, ]>
>>> Article.objects.all()
, , , ]>
>>> a2.publications.all()
]>
批量删除一些文章 - 引用删除的对象应该去:
>>> q = Article.objects.filter(headline__startswith='Django')
>>> print(q)
]>
>>> q.delete()
在delete()
之后,需要清除QuerySet
缓存,并且引用的对象应该消失:
>>> print(q)
>>> p1.article_set.all()
]>
2017年9月6日