Typically autoload paths configured in application.rb will be like this:

config.autoload_paths += %W(lib)

Recently we faced a problem where we were adding another root level folder app to autoload_paths, and using one of utility classes inside that folder. Everthing worked fine in development, but not in production. We started seeing strange errors while running migrations:

cannot load such file -- app/subdir1/utility_class

Inspecting $LOAD_PATH revealed that there is no absolute path for app folder. Appending autoload_path with absolute path solved this issue.

config.autoload_paths += [ Rails.root.join("app") ]