module ActiveSupport::Dependencies::RequireDependency

def require_dependency(filename)

+:classic+.
should call +require_dependency+ where needed in case the runtime mode is
Engines that do not control the mode in which their parent application runs

Just refer to classes and modules normally.
match Ruby's and you do not need to be defensive with load order anymore.
Warning: This method is obsolete. The semantics of the autoloader 
def require_dependency(filename)
  filename = filename.to_path if filename.respond_to?(:to_path)
  unless filename.is_a?(String)
    raise ArgumentError, "the file name must be either a String or implement #to_path -- you passed #{filename.inspect}"
  end
  if abspath = ActiveSupport::Dependencies.search_for_file(filename)
    require abspath
  else
    require filename
  end
end