Someone at Real World asked me about maintaining shared code between projects in Real Studio. He was using external project items (in XML format) and that makes it difficult to work with source control since the XML format isn’t really designed for that. We have a lot of code that we share between projects, both code that we’ve written like ActiveRecord and open source code or libraries that we’ve purchased.
We don’t try to share the code automatically. Instead we keep shared code in a separate project (ideally with testing code, although we’re not at 100%). That code is separately versioned and we update individual projects by copying the new version into the project’s that use it.
In our experience this works much better than trying to share code automatically. We work on a lot of projects and sometimes a project won’t have changes for months or even years. If the shared code has changed in the meantime, we don’t want to go back to the project that uses it and find that it may have bugs that were introduced by changes to the shared code.
This applies to other language also by the way. I can’t see myself ever having code shared between separate projects (with the exception of header files in C or C++).
If you search for vendor branches you can find some more information that’s specific to Subversion. We don’t use the vendor branch pattern ourselves but it’s worth knowing about.