首先引用官網(wǎng)文檔的代碼引入外鍵和多鍵

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 __str__(self):
        return self.title

獲取數(shù)據(jù)庫對(duì)象的一般特定字段的值只需直接使用屬性,但是ForeignKey 或 ManyToManyField的關(guān)聯(lián)對(duì)象字段,它們的作用稍有不同。

訪問外鍵(Foreign Key)值

當(dāng)你獲取一個(gè)ForeignKey 字段時(shí),你會(huì)得到相關(guān)的數(shù)據(jù)模型對(duì)象。

>>> b = Book.objects.get(id=50)
>>> b.publisher
<Publisher: Apress Publishing>
>>> b.publisher.website
u'http://www.apress.com/'

對(duì)于用ForeignKey 來定義的關(guān)系來說，在關(guān)系的另一端也能反向的追溯回來， 通過一個(gè)publisher 對(duì)象，直接獲取 books ，用 publisher.book_set.all() ，如下：

>>> p = Publisher.objects.get(name='Apress Publishing')
>>> p.book_set.all()
[<Book: The Django Book>, <Book: Dive Into Python>, ...]

實(shí)際上，book_set 只是一個(gè) QuerySet（參考第5章的介紹），所以它可以像QuerySet一樣,能實(shí)現(xiàn)數(shù)據(jù)過濾和分切,屬性名稱book_set是由模型名稱的小寫(如book)加_set組成的。

>>> p = Publisher.objects.get(name='Apress Publishing')
>>> p.book_set.filter(name__icontains='django')
[<Book: The Django Book>, <Book: Pro Django>]

訪問多對(duì)多值(Many-to-Many Values)

多對(duì)多和外鍵工作方式相同，只不過我們處理的是QuerySet而不是模型實(shí)例

>>> 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')
[]

反向查詢也可以。 要查看一個(gè)作者的所有書籍,使用author.book_set ,這里,就像使用 ForeignKey字段一樣，屬性名book_set是在數(shù)據(jù)模型(model)名后追加_set。

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