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 models

    class 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.name

    class 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。