在Python的Django框架中更新数据库数据的方法
先使用一些关键参数创建对象实例，如下：
>>> p = Publisher(name='Apress', ... address='2855 Telegraph Ave.', ... city='Berkeley', ... state_province='CA', ... country='U.S.A.', ... website='http://www.apress.com/')
这个对象实例并 没有 对数据库做修改。 在调用`` save()`` 方法之前，记录并没有保存至数据库，像这样：
>>> p.save()
在SQL里，这大致可以转换成这样：
INSERT INTO books_publisher
(name, address, city, state_province, country, website)
VALUES
('Apress', '2855 Telegraph Ave.', 'Berkeley', 'CA',
'U.S.A.', 'http://www.apress.com/');
因为 Publisher 模型有一个自动增加的主键 id ，所以第一次调用 save() 还多做了一件事： 计算这个主键的值并把它赋值给这个对象实例：
>>> p.id 52 # this will differ based on your own data
接下来再调用 save() 将不会创建新的记录，而只是修改记录内容（也就是 执行 UPDATE SQL语句，而不是 INSERT 语句）：
>>> p.name = 'Apress Publishing' >>> p.save()
前面执行的 save() 相当于下面的SQL语句：
UPDATE books_publisher SET name = 'Apress Publishing', address = '2855 Telegraph Ave.', city = 'Berkeley', state_province = 'CA', country = 'U.S.A.', website = 'http://www.apress.com' WHERE id = 52;
注意，并不是只更新修改过的那个字段，所有的字段都会被更新。 这个操作有可能引起竞态条件，这取决于你的应用程序。 请参阅后面的“更新多个对象”小节以了解如何实现这种轻量的修改（只修改对象的部分字段）。
UPDATE books_publisher SET name = 'Apress Publishing' WHERE id=52;
选择对象
当然，创建新的数据库，并更新之中的数据是必要的，但是，对于 Web 应用程序来说，更多的时候是在检索查询数据库。 我们已经知道如何从一个给定的模型中取出所有记录：
>>> Publisher.objects.all() [<Publisher: Apress>, <Publisher: O'Reilly>]
这相当于这个SQL语句：
SELECT id, name, address, city, state_province, country, website FROM books_publisher;
注意
注意到Django在选择所有数据时并没有使用 SELECT* ，而是显式列出了所有字段。 设计的时候就是这样： SELECT* 会更慢，而且最重要的是列出所有字段遵循了Python 界的一个信条： 明言胜于暗示。
有关Python之禅(戒律) :-），在Python提示行输入 import this 试试看。
让我们来仔细看看 Publisher.objects.all() 这行的每个部分：
- 首先，我们有一个已定义的模型 Publisher 。没什么好奇怪的： 你想要查找数据， 你就用模型来获得数据。
- 然后，是objects属性。 它被称为管理器，我们将在第10章中详细讨论它。 目前，我们只需了解管理器管理着所有针对数据包含、还有最重要的数据查询的表格级操作。
- 所有的模型都自动拥有一个 objects 管理器；你可以在想要查找数据时使用它。
- 最后，还有 all() 方法。这个方法返回返回数据库中所有的记录。 尽管这个对象 看起来 象一个列表（list），它实际是一个 QuerySet 对象， 这个对象是数据库中一些记录的集合。
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