autoload_and_fakefs.rb

# fakefs will break autoloading in your tests. You can correct this by
# eval'ing the modules you care about before requiring fakefs. One way
# to do this would be to take the top-level module (or class) that
# contains modules (or classes) to be autoloaded and eval all of them
# in.

def autoload_all(namespace)
  namespace.constants.each do |c|
    const = namespace.module_eval c
    autoload_all(const) if const.is_a? Module
  end
end

autoload_all MyModuleOrClass

require 'fakefs'
# .. the rest of your test code