Rails 3.2 + pg gem：加载路径问题

Question

我有两个用Rails创建的应用程序，“旧的”和新的

    $ rails new blog -d postgresql

当我尝试启动服务器时，它可以在“旧”应用程序上正常运行：

    $ rails s -p 2000
    => Booting WEBrick
    => Rails 3.2.8 application starting in development on http://0.0.0.0:2000
    => Call with -d to detach
    => Ctrl-C to shutdown server
    [2012-09-17 16:28:48] INFO  WEBrick 1.3.1
    [2012-09-17 16:28:48] INFO  ruby 1.9.3 (2012-04-20) [x86_64-linux]
    [2012-09-17 16:28:48] INFO  WEBrick::HTTPServer#start: pid=9377 port=2000

但对于“新”应用程序失败：/

    $ rails s -p 2000
    /usr/local/share/gems/gems/pg-0.14.1/lib/pg.rb:4:in `require': cannot load such file -- pg_ext (LoadError)
    from /usr/local/share/gems/gems/pg-0.14.1/lib/pg.rb:4:in `<top (required)>'
    from /usr/local/share/gems/gems/bundler-1.1.5/lib/bundler/runtime.rb:68:in `require'
    from /usr/local/share/gems/gems/bundler-1.1.5/lib/bundler/runtime.rb:68:in `block (2 levels) in require'
    from /usr/local/share/gems/gems/bundler-1.1.5/lib/bundler/runtime.rb:66:in `each'
    from /usr/local/share/gems/gems/bundler-1.1.5/lib/bundler/runtime.rb:66:in `block in require'
    from /usr/local/share/gems/gems/bundler-1.1.5/lib/bundler/runtime.rb:55:in `each'
    from /usr/local/share/gems/gems/bundler-1.1.5/lib/bundler/runtime.rb:55:in `require'
    from /usr/local/share/gems/gems/bundler-1.1.5/lib/bundler.rb:119:in `require'
    from /home/alex/tmp/RoR/blog/config/application.rb:7:in `<top (required)>'
    from /usr/local/share/gems/gems/railties-3.2.8/lib/rails/commands.rb:53:in `require'
    from /usr/local/share/gems/gems/railties-3.2.8/lib/rails/commands.rb:53:in `block in <top (required)>'
    from /usr/local/share/gems/gems/railties-3.2.8/lib/rails/commands.rb:50:in `tap'
    from /usr/local/share/gems/gems/railties-3.2.8/lib/rails/commands.rb:50:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'

如果我将pg_ext.so复制或符号链接到模块路径，则可以修复：

    # ln -s /usr/local/share/gems/gems/pg-0.14.1/ext/pg_ext.so /usr/local/share/gems/gems/pg-0.14.1/lib

或者我可以直接将要求添加到boot.rb中：

    require 'rubygems'
    require 'pg'

但这不是可接受的解决方案，我想知道为什么它适用于第一个应用程序，为什么它不适用于新应用程序？ 我花了一整天比较配置文件，但没有成功。

Answer 1

升级到Fedora 18后，我遇到了同样的问题。

pg_ext是在运行gem update时构建的。

前提条件是必须安装postgresql-devel。