今天我自己也碰上了这个，决定下潜.

Just ran into this myself today and decided to dive deep.

在ActiveSupport::Dependencies.autoload_paths中看不到要添加到config/application.rb中的config.autoload_paths中的路径的原因是，在初始化应用程序之前，它们不会被复制.请参见railties宝石中的rails/engine.rb:

The reason you don't see in ActiveSupport::Dependencies.autoload_paths the paths you're adding to config.autoload_paths in config/application.rb is that they aren't copied over until the application is initialized. See rails/engine.rb in the railties gem:

module Rails
  class Engine < Railtie
    …

    # Set the paths from which Rails will automatically load source files,
    # and the load_once paths.
    #
    # This needs to be an initializer, since it needs to run once
    # per engine and get the engine as a block parameter
    initializer :set_autoload_paths, :before => :bootstrap_hook do |app|
      ActiveSupport::Dependencies.autoload_paths.unshift(*_all_autoload_paths)
      ActiveSupport::Dependencies.autoload_once_paths.unshift(*_all_autoload_once_paths)

      # Freeze so future modifications will fail rather than do nothing mysteriously
      config.autoload_paths.freeze
      config.eager_load_paths.freeze
      config.autoload_once_paths.freeze
    end

    …

    def _all_autoload_paths
      @_all_autoload_paths ||= (config.autoload_paths + config.eager_load_paths + config.autoload_once_paths).uniq
    end

    …
  end
end

您是不是偶然地尝试从config/application.rb内部甚至更早地从需要config/application.rb的脚本或模块中调用MyClass?如果是这样，您将必须明确要求定义MyClass的文件，例如:

Were you by any chance trying to invoke MyClass from within config/application.rb or, even earlier, from a script or module that requires config/application.rb? If so, you'll have to explicitly require the file defining MyClass, e.g.:

require File.expand_path('../../somefolder/my_class',  __FILE__)
# now use MyClass