通过数据库向Django模型添加字段的示例
首先借用书本(book)的这个数据模型：
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
添加字段
当要向一个产品设置表(或者说是model)添加一个字段的时候，要使用的技巧是利用Django不关心表里是否包含model里所没有的列的特性。 策略就是现在数据库里加入字段，然后同步Django的模型以包含新字段。
然而 这里有一个鸡生蛋蛋生鸡的问题 ,由于要想了解新增列的SQL语句，你需要使用Django的 manage.py sqlall命令进行查看 ,而这又需要字段已经在模型里存在了。 (注意:你并 不是非得使用与Django相同的SQL语句创建新的字段，但是这样做确实是一个好主意 ,它能让一切都保持同步。)
这个鸡-蛋的问题的解决方法是在开发者环境里而不是发布环境里实现这个变化。 (你正使用的是测试/开发环境，对吧?)下面是具体的实施步骤。
首先，进入开发环境(也就是说，不是在发布环境里)：
在你的模型里添加字段。
运行 manage.py sqlall [yourapp] 来测试模型新的 CREATE TABLE 语句。 注意为新字段的列定义。
开启你的数据库的交互命令界面(比如, psql 或mysql , 或者可以使用 manage.py dbshell )。 执行 ALTER TABLE 语句来添加新列。
使用Python的manage.py shell，通过导入模型和选中表单(例如， MyModel.objects.all()[:5] )来验证新的字段是否被正确的添加 ,如果一切顺利,所有的语句都不会报错。
然后在你的产品服务器上再实施一遍这些步骤。
启动数据库的交互界面。
执行在开发环境步骤中，第三步的ALTER TABLE语句。
将新的字段加入到模型中。 如果你使用了某种版本控制工具，并且在第一步中，已经提交了你在开发环境上的修改，现在，可以在生产环境中更新你的代码了（例如，如果你使用Subversion，执行svn update。
重新启动Web server，使修改生效。
让我们实践下，比如添加一个num_pages字段到第五章中Book模型。首先，我们会把开发环境中的模型改成如下形式：
class Book(models.Model):
title = models.CharField(max_length=100)
authors = models.ManyToManyField(Author)
publisher = models.ForeignKey(Publisher)
publication_date = models.DateField()
**num_pages = models.IntegerField(blank=True, null=True)**
def __unicode__(self):
return self.title
然后，我们运行命令manage.py sqlall books 来查看CREATE TABLE语句。 语句的具体内容取决与你所使用的数据库， 大概是这个样子：
CREATE TABLE "books_book" (
"id" serial NOT NULL PRIMARY KEY,
"title" varchar(100) NOT NULL,
"publisher_id" integer NOT NULL REFERENCES "books_publisher" ("id"),
"publication_date" date NOT NULL,
"num_pages" integer NULL
);
新加的字段被这样表示：
"num_pages" integer NULL
接下来，我们要在开发环境上运行数据库客户端，如果是PostgreSQL，运行 psql,，然后，我执行如下语句。
ALTER TABLE books_book ADD COLUMN num_pages integer;
添加 非NULL 字段
这里有个微妙之处值得一提。 在我们添加字段num_pages的时候，我们使用了 blank=True 和 null=True 选项。 这是因为在我们第一次创建它的时候，这个数据库字段会含有空值。
然而，想要添加不能含有空值的字段也是可以的。 要想实现这样的效果，你必须先创建 NULL 型的字段，然后将该字段的值填充为某个默认值，然后再将该字段改为 NOT NULL 型。 例如：
BEGIN; ALTER TABLE books_book ADD COLUMN num_pages integer; UPDATE books_book SET num_pages=0; ALTER TABLE books_book ALTER COLUMN num_pages SET NOT NULL; COMMIT;
如果你这样做，记得你不要在模型中添加 blank=True 和 null=True 选项。
执行ALTER TABLE之后，我们要验证一下修改结果是否正确。启动python并执行下面的代码：
>>> from mysite.books.models import Book >>> Book.objects.all()[:5]
如果没有异常发生，我们将切换到生产服务器，然后在生产环境的数据库中执行命令ALTER TABLE 然后我们更新生产环境中的模型，最后重启web服务器。
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