故障现象：
[root@cloudmaster gem]# gem install rails -l
ERROR: Error installing rails:
activemodel requires builder (~> 3.0.0, runtime)
[root@cloudmaster gem]#
故障分析：
gem包使用本地安装，而本地builder-3.1.4.gem版本大于3.0.0，按提示应该不是此问题；尝试其他方法解决都行不通，故找来另一台机器直接gem install rails一下，之后gem list结果如下：
[root@ezCloud ~]# gem install rails
Successfully installed rails-3.2.9
1 gem installed
Installing ri documentation for rails-3.2.9...
Installing RDoc documentation for rails-3.2.9...
[root@ezCloud ~]# gem list
*** LOCAL GEMS ***
actionmailer (3.2.9)
actionpack (3.2.9)
activemodel (3.2.9)
activerecord (3.2.9)
activeresource (3.2.9)
activesupport (3.2.9)
arel (3.0.2)
builder (3.0.4)
bundler (1.2.3)
daemons (1.1.9)
erubis (2.7.0)
eventmachine (1.0.0)
headers (0.0.1)
hike (1.2.1)
i18n (0.6.1)
journey (1.0.4)
json (1.7.5, 1.4.6)
libraries (0.1.6)
mail (2.4.4)
mime-types (1.19)
multi_json (1.5.0)
nokogiri (1.5.6)
polyglot (0.3.3)
rack (1.4.1)
rack-cache (1.2)
rack-protection (1.3.2)
rack-ssl (1.3.2)
rack-test (0.6.2)
rails (3.2.9)
railties (3.2.9)
rake (10.0.3)
rdoc (3.12)
sequel (3.42.0)
sinatra (1.3.3)
sinatra-sequel (0.9.0)
sprockets (2.2.2)
thin (1.5.0)
thor (0.16.0)
tilt (1.3.3)
treetop (1.4.12)
tzinfo (0.3.35)
[root@ezCloud ~]#
结果发现builder为3.0.4，我的ruby版本是1.8.7，可能是本地的builder版本过高缘故，下载builder-3.0.4并安装：
[root@cloudmaster gem]# wget http://tokyo-m.rubygems.org/gems/builder-3.0.4.gem
[root@cloudmaster gem]# gem install builder-3.0.4.gem -l
Successfully installed builder-3.0.4
1 gem installed
Installing ri documentation for builder-3.0.4...
Installing RDoc documentation for builder-3.0.4...
[root@cloudmaster gem]# gem install rails -l
ERROR: Error installing rails:
actionmailer requires mail (~> 2.4.4, runtime) /*要求mail的版本是2.4.4*/
[root@cloudmaster gem]# wget http://tokyo-m.rubygems.org/gems/mail-2.4.4.gem
再次安装rails，安装成功；
[root@cloudmaster gem]# gem install rails -l
Successfully installed mime-types-1.19
Successfully installed polyglot-0.3.3
Successfully installed treetop-1.4.12
Successfully installed mail-2.4.4
Successfully installed actionmailer-3.2.9
Successfully installed rack-ssl-1.3.2
Successfully installed thor-0.16.0
Successfully installed railties-3.2.9
Successfully installed ………………
gem download:
http://rubygems.org/gems/sequel
gem install:
gem install sequel -l