Render CSV from Rails respond_with

csv_renderer.rb

# Use with render :csv => foo and respond_with @foo if @foo responds to to_csv
# see ActionController::Responder#to_format and #api_behavior
ActionController::Renderers.add(:csv) do |obj, options|
  csv = if obj.respond_to?(:to_csv)
          obj.to_csv(options[:csv_options] || {})
        else
          obj
        end

  headers['Content-Disposition'] = if options[:filename].blank?
                                     'attachment'
                                   else
                                     'attachment;'\
                                     " filename=#{options[:filename]}"
                                   end

  self.content_type ||= Mime::CSV

  csv
end