Django, one-to-many, many-to-many

1、定义关系

定义三个表，Publisher，Book，Author
一个作者有姓，有名及email地址。
出版商有名称，地址，所在城市、省，国家，网站。
书籍有书名和出版日期。它有一个或多个作者（和作者是多对多的关联关系[many-to-many]），只有一个出版商（和出版商是一对多的关联关系[one-to-many]，也被称作外键[foreign key]）

2、定义model

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()

class Author(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=40)
    email = models.EmailField()

class Book(models.Model):
    title = models.CharField(max_length=100)
    authors = models.ManyToManyField(Author)
    publisher = models.ForeignKey(Publisher)
    publication_date = models.DateField()

3、基本数据库访问

　　当你使用Django modle API创建对象时Django并未将对象保存至数据库内，除非你调用save() 方法

>>> from books.models import Publisher
>>> p1 = Publisher(name='Apress', address='2855 Telegraph Avenue',
...     city='Berkeley', state_province='CA', country='U.S.A.',
...     website='http://www.apress.com/')
>>> p1.save()
>>> p2 = Publisher(name="O'Reilly", address='10 Fawcett St.',
...     city='Cambridge', state_province='MA', country='U.S.A.',
...     website='http://www.oreilly.com/')
>>> p2.save()
>>> publisher_list = Publisher.objects.all()
>>> publisher_list
[, ]

　　如果需要一步完成对象的创建与存储至数据库，就使用objects.create() 方法

>>> p1 = Publisher.objects.create(name='Apress',
...     address='2855 Telegraph Avenue',
...     city='Berkeley', state_province='CA', country='U.S.A.',
...     website='http://www.apress.com/')
>>> p2 = Publisher.objects.create(name="O'Reilly",
...     address='10 Fawcett St.', city='Cambridge',
...     state_province='MA', country='U.S.A.',
...     website='http://www.oreilly.com/')
>>> publisher_list = Publisher.objects.all()
>>> publisher_list

　　数据过滤

>>> Publisher.objects.filter(name='Apress')
[<Publisher: Apress>]

>>> Publisher.objects.filter(country="U.S.A.", state_province="CA")
[<Publisher: Apress>]

>>> Publisher.objects.filter(name__contains="press")
[<Publisher: Apress>]

　　contains部分会被Django翻译成LIKE语句

　　获取单个对象

>>> Publisher.objects.get(name="Apress")
<Publisher: Apress>

　　排序

>>> Publisher.objects.order_by("name")
[<Publisher: Apress>, <Publisher: O'Reilly>]

　　访问外键

>>> b = Book.objects.get(id=50)
>>> b.publisher

>>> b.publisher.website
u'http://www.apress.com/'

　　访问多对多

>>> 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

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