```ruby
require 'sprockets/railtie'

module Less  
  module Rails
    class Railtie < ::Rails::Railtie
      config.less = ActiveSupport::OrderedOptions.new
      config.less.paths = []
      config.less.compress = false
      config.app_generators.stylesheet_engine :less

      config.before_initialize do |app|
        require 'less'
        require 'less-rails'
        Sprockets::Engines #force autoloading
        Sprockets.register_engine '.less', LessTemplate
      end

      initializer 'less-rails.before.load_config_initializers', :before => :load_config_initializers, :group => :all do |app|
        app.assets.register_preprocessor 'text/css', ImportProcessor
        Sprockets.register_preprocessor 'text/css', ImportProcessor if Sprockets.respond_to?('register_preprocessor')

        config.assets.configure do |env|
          env.context_class.class_eval do
            class_attribute :less_config
            self.less_config = app.config.less
          end
        end
      end

      initializer 'less-rails.after.append_assets_path', :after => :append_assets_path, :group => :all do |app|
        assets_stylesheet_paths = app.config.assets.paths.select { |p| p && p.to_s.ends_with?('stylesheets') }
        app.config.less.paths.unshift(*assets_stylesheet_paths)
      end
      
      initializer 'less-rails.setup_compression', :group => :all do |app|
        config.less.compress = app.config.assets.compress
      end
      
    end
  end
end

```

Fix Undefined MethodError at app.assets.register_preprocessor for sprockets3.X
user
```ruby
require 'sprockets/railtie'

module Less  
  module Rails
    class Railtie < ::Rails::Railtie
      config.less = ActiveSupport::OrderedOptions.new
      config.less.paths = []
      config.less.compress = false
      config.app_generators.stylesheet_engine :less

      config.before_initialize do |app|
        require 'less'
        require 'less-rails'
        Sprockets::Engines #force autoloading
        Sprockets.register_engine '.less', LessTemplate
      end

      initializer 'less-rails.before.load_config_initializers', :before => :load_config_initializers, :group => :all do |app|
        sprockets_env = app.assets || Sprockets
        sprockets_env.register_preprocessor 'text/css', ImportProcessor

        config.assets.configure do |env|
          env.context_class.class_eval do
            class_attribute :less_config
            self.less_config = app.config.less
          end
        end
      end

      initializer 'less-rails.after.append_assets_path', :after => :append_assets_path, :group => :all do |app|
        assets_stylesheet_paths = app.config.assets.paths.select { |p| p && p.to_s.ends_with?('stylesheets') }
        app.config.less.paths.unshift(*assets_stylesheet_paths)
      end
      
      initializer 'less-rails.setup_compression', :group => :all do |app|
        config.less.compress = app.config.assets.compress
      end
      
    end
  end
end

```
assistant