Upgrade from Rails 3.2 to Rails 4

The asset pipeline has changed a bit, so you'll need to upgrade those gems. I had to use a fork of ransack in the context of active_admin, but you might be fine.

I'd recommend that you create a branch, bump to Rails 4, and see what happens. It took me a day or two to upgrade from 3.2 to 4 a couple weeks ago on a fairly small (but production-with-customers) application. The upgrade guide is pretty solid.

I uploaded your gemfile to Ready for Rails 4, and it appears that you only have a couple gems that are not ready and one gem that is unknown. For some of the gems listed that do not have notes, I would suggest checking out their GitHub page (if they have one), and see if the gem has been updated recently on rubygems, just to confirm whether or not the gem works.

This brief handbook was worth every cent in our recent 3.2 to 4 migration.

https://leanpub.com/upgradetorails4

https://github.com/alindeman/upgradingtorails4

It lists in details how to handle gem upgrades, as well as individual details (with clear examples) of what's changed and how to manage your current Rails app through the upgrade.

To address your gem-specific concerns: Almost all our 40+ gems, except perhaps 5, had active either Rails 4 releases (or Github branches for Rails 4), or worked just fine with Rails 4 when left intact.

Having a healthy unit/functional test suite in your codebase would be vital for your confidence in the upgrade being successful.