How to force rack to work around the usual “You have already activated rack…” bug?

Question

This is a common question, but none of the answers seem to solve the issue. I get the usual: You have already activated rack 1.4.1, but your Gemfile requires rack 1.3.

Answer 1

Sometimes all you need to do is just install the gem.

I had this problem on openshift and gone to the project dir:

$ rhc ssh APP_NAME

$ cd app-root

$ gem install GEM_NAME

after that the app started normally.