My gem depends on nokogiri ~> 1.5.0. I'd like to keep that dependency since this version has less compile-time dependencies and generally installs without problems. nokogiri 1.6.x seems to have more problems installing. So while I'd like to support nokogiri 1.6 if is is already installed, I don't want to make it the default dependency.
So: Prefer nokogiri ~> 1.5.0 but use 1.6 if it's present.
Is it possible to express that as a dependency in my Gemfile or gemspec?
You can specify a comma-separated list of version specifiers in your gemspec. They are ANDed together.
spec.add_runtime_dependency "nokogiri", ">= 1.5.0", "< 1.7"
This says that any 1.5.x or 1.6.x version is compatible.
Rubygems doesn't have any notion of a "preferred" version. Bundler and gem will generally try to install the latest version available that satisfies all of the constraints.
See http://guides.rubygems.org/patterns/#declaring-dependencies for details.
