How to debug a ruby gem?
I'm not sure if there is a better way to do, but I often do like this:
Check gem path
bundle show [gem_name]
If you are Mac user, it gets copied to your clipboard
bundle show [gem_name] | pbcopy
Open your gem directory (Vim is my editor)
vim /path/to/your/gem
Find your file and add a breakpoint or inspect variables
Don't forget "restart your rails app" to apply new code
:D