calling_render.rb

def generate_daily_pos_report
      # create a fake request
      fake_request = Merb::Request.new({})
      # init the controller
      controller = Purchasing::Reports.new(fake_request)
      # gets all the pos created on today
      pos = PurchaseOrder.all(:deleted => false, :po_date => date)

      results = "No purchase orders created on #{date}"
      if pos.size > 0
        ## set the instance varialbe for the html.erb, :@pos see report/deaily_pos.html.erb
        controller.instance_variable_set(:@pos, pos)
        # gets the html string
        results = controller.render :_daily_pos_results, :layout => :email
      end

      # if you want to re-use the logic in controll, you have to use _dispatch
      # but this is not a good solution, you have embed in some extra logic to 
      # handle the render method in controller, the best way to re-use the logic
      # in controller is to move the logic out of controller, put it some services
      # layer, then you can re-use it.
      # result = controller._dispatch(:daily_pos)

      file = purchasing_dir + "/#{date}.html"
      File.open(file, 'w') {|f| f.puts results}
      file
end