Django的数据模型访问多对多键值的方法

这里先来借用一个书本(book)的数据模型作为例子：

from django.db import modelsclass Publisher(models.Model):  name = models.CharField(max_length=30)  address = models.CharField(max_length=50)  city = models.CharField(max_length=60)  state_province = models.CharField(max_length=30)  country = models.CharField(max_length=50)  website = models.URLField()  def __unicode__(self):    return self.nameclass Author(models.Model):  first_name = models.CharField(max_length=30)  last_name = models.CharField(max_length=40)  email = models.EmailField()  def __unicode__(self):    return u'%s %s' % (self.first_name, self.last_name)class Book(models.Model):  title = models.CharField(max_length=100)  authors = models.ManyToManyField(Author)  publisher = models.ForeignKey(Publisher)  publication_date = models.DateField()  def __unicode__(self):    return self.title

访问多对多值(Many-to-Many Values)

多对多和外键工作方式相同，只不过我们处理的是QuerySet而不是模型实例。 例如,这里是如何查看书籍的作者：

>>> b = Book.objects.get(id=50)>>> b.authors.all()[<Author: Adrian Holovaty>, <Author: Jacob Kaplan-Moss>]>>> b.authors.filter(first_name='Adrian')[<Author: Adrian Holovaty>]>>> b.authors.filter(first_name='Adam')[]

反向查询也可以。 要查看一个作者的所有书籍,使用author.book_set ,就如这样:

>>> a = Author.objects.get(first_name='Adrian', last_name='Holovaty')>>> a.book_set.all()[<Book: The Django Book>, <Book: Adrian's Other Book>]

这里,就像使用 ForeignKey字段一样，属性名book_set是在数据模型(model)名后追加_set。