load_missing_constant

# File activesupport/lib/active_support/dependencies.rb, line 487
    def load_missing_constant(from_mod, const_name)
      log_call from_mod, const_name

      unless qualified_const_defined?(from_mod.name) && Inflector.constantize(from_mod.name).equal?(from_mod)
        raise ArgumentError, "A copy of #{from_mod} has been removed from the module tree but is still active!"
      end

      raise NameError, "#{from_mod} is not missing constant #{const_name}!" if local_const_defined?(from_mod, const_name)

      qualified_name = qualified_name_for from_mod, const_name
      path_suffix = qualified_name.underscore

      file_path = search_for_file(path_suffix)

      if file_path && ! loaded.include?(File.expand_path(file_path).sub(/\.rb\z/, '')) # We found a matching file to load
        require_or_load file_path
        raise LoadError, "Expected #{file_path} to define #{qualified_name}" unless local_const_defined?(from_mod, const_name)
        return from_mod.const_get(const_name)
      elsif mod = autoload_module!(from_mod, const_name, qualified_name, path_suffix)
        return mod
      elsif (parent = from_mod.parent) && parent != from_mod &&
            ! from_mod.parents.any? { |p| local_const_defined?(p, const_name) }
        # If our parents do not have a constant named +const_name+ then we are free
        # to attempt to load upwards. If they do have such a constant, then this
        # const_missing must be due to from_mod::const_name, which should not
        # return constants from from_mod's parents.
        begin
          return parent.const_missing(const_name)
        rescue NameError => e
          raise unless e.missing_name? qualified_name_for(parent, const_name)
        end
      end

      raise NameError,
            "uninitialized constant #{qualified_name}",
            caller.reject {|l| l.starts_with? __FILE__ }
    end