在Rails下定义常量和全局变量的方法如下，如果你希望任何环境（开发，测试，产品）下都可以访问的话，则定义在config/environment.rb中
如：
BOOLEN_TRUE=1
如果你希望你的全局变量根据环境来决定，则根据需要在下面的环境中配置
/config/environments/development.rb
/config/environments/test.rb
/config/environments/production.rb
注意，
1> 常量和全局变量必须全部为大写；
2> 设置了全局常量和变量后需重新启动服务器以加载新的设置。
PS. english version
I was recently faced with a common problem that I wanted to share. During development there are times you need to specifiy a global or constant variable that will be shared throughout the entire application. In ColdFusion you would normally place something like this in Application.cfm. In Rails I was uncertain how exactly to approach this. At first I thought an application.rb file made sense, but the more I thought about it I decided to take advantage of the config files.
Since my global variable was one that would be consistent regardless of environment (dev, test, and prod) I decided to place it in config/environment.rb. I placed this under the RAILS_GEM_VERSION constant:
# Specifies gem version of Rails to use when vendor/rails is not present
RAILS_GEM_VERSION = ‘1.1.2′
BASE_URL = ‘http://mydomain.com/’
Now referencing this in any view can be done similar to any other variable:
<%= BASE_URL %>
If you need to specify globals that are specific to environment then I recommend placing them in their appropriate config file:
/config/environments/development.rb
/config/environments/test.rb
/config/environments/production.rb
From: http://www.db75.com/new_blog/?p=262
