thin down json responses in rails3

Question

I want to get my Rails3 app to respond to requests for some assets with json. For my application it would be inappropriate to send across all the information relating to a asset though, is there some way to specify fields? I know there's a way in Rails2. Here's some code:

  def index
    @directories = Directory.all

    respond_to do |format|
      format.html # index.html.erb
      format.xml  { render :xml => @directories }
      format.json { render :json => @directories }
    end
  end

Answer 1

Answer is a bit outdated however I suppose it will work render :json does not accept options