Rails: Missing or uninitialized constant

If you run rake task or something else, and get: Missing or uninitialized constant exception, then check your config/environments/development.rb for config.threadsafe!

From guides.rubyonrails.org:

Threadsafe operation is incompatible with the normal workings of development mode Rails. In particular, automatic dependency loading and class reloading are automatically disabled when you call config.threadsafe!.

So, never set config.threadsafe! for development environment :)