as_json not honouring the options hash

I am new to ROR. I have been going through the article regarding as_json Rails to_json or as_json? - Jonathan Julian

but i could not make as_json work with options hash . When i am using as_json with options, it is not honoring the options and returning the entire object.So a call like render :json => {:user.as_json(:only => [:email] , :methods => [:getName]} does not work as expected and returns the whole user object and disregards the :methods and :only. Can anyone suggest any solution for this?