According to this ticket #1905, app/helpers in rails plugins-engines doesn't get mixed into the ActionView::Base, and so the methods are not available in the views.

The reason for this is, that the "all_application_helpers" method in ActionController::Helper only returns the helper modules in RAILS_ROOT/app/helper and not those in vendor/plugins/**/app/helpers:

   1  #file: action_controller/helpers.rb
   2  #line: 219
   3          # Extract helper names from files in app/helpers/**/*.rb
   4    def all_application_helpers
   5      extract = /^#{Regexp.quote(helpers_dir)}\/?(.*)_helper.rb$/
   6      Dir["#{helpers_dir}/**/*_helper.rb"].map { |file| file.sub extract, '\1' }
   7    end
   8  

So when you say

   1  helper :all

in your controller, the helper-methods of your plugin are not available.

BUT, ... all the helper modules of your plugin are added to the load_path, so a quick fix for this problem would be to load the helper-module manually

   1  # for every helper file in your engines:
   2  helper :my_plugin_helper

Then the corresponding module-name is "constantized" and included into ActionView::Base by the "add_template_helper" method:

   1  #file: action_controller/helpers.rb
   2  #line: 75
   3    def add_template_helper(helper_module) #:nodoc:
   4      master_helper_module.module_eval { include helper_module }
   5    end

..and all helper-methods of your plugin/engine are available in your views.