uninitialized constant HelloMessageController::HelloMessageApi

[Magudamudi]

Hi,
I practice some examples but i couldn't run it well

hello_message_c ontroller.rb:

class HelloMessageController < ApplicationController
   
  web_service_api HelloMessageApi
    wsdl_service_name 'hello_message'
        web_service_scaffold :invoke

  def hello_message(firstname, lastname)
    return "Hello" + firstname+" "+lastname
  end

end

Hello_message_a pi.rb:

class HelloMessageApi < ActionWebService::API::Base
  api_method :hello_message, expects => [{:firstname=>:string},
    {:lastname=>string}], :returns=>[:string]
end

when i open localhost:3000/hello_message/invoke
i got the following error:

NameError in Hello messageControll er#invoke

uninitialized constant HelloMessageCon troller::HelloM essageApi

Any one help me why this error occur and how can we overcome this error?

Thanks in Advance!