In config/environments/development.rb set:
config.assets.prefix = "/assets_dev"
so that in development mode Rails will look there (but it will not find anything, as you will not compile assets in development (this is indeed what you are trying to do — not compile assets)).
When precompiling for production, use
RAILS_ENV=production rake assets:precompile
so it compiles into the default assets folder, public/assets.