I really don't like this solution. To fork and link to your own version of Rails just to get something like Array#sum is not sustainable. Yes, Bundler makes tho

I really don't like this solution. To fork and link to your own version of Rails just to get something like Array#sum is not sustainable. Yes, Bundler makes those first steps easy, but maintenance doesn't seem particularly fun after that (especially for a project as active as Rails). Pull requests, if done right, are the real way to go. If the pull request isn't accepted (but the maintainer is an active one), maybe you should ask yourself why you need to implement this change in the first place. (Or why you need to implement it this way.) That said, refinements (aka, monkey-patching without consequences) should show up soon and can solve this problem for anyone who upgrades to Ruby 2.