Why do you want to use Xcode for Rails? Of course I'm not saying you can't, but in my experience Xcode is not that great for anything outside of C/C++/Objective-C development. It may support other languages and frameworks, but it's really built around a C-based, native desktop application target. What you could do is set an external app as the editor for code files. For example, TextWrangler or TextMate, which do have good support for Ruby/Rails syntax. Or you could just use those for the whole project and cut Xcode out.