orders_controller.rb

class OrdersController < ApplicationController

  def index
    ...
    respond_to do |format|
      format.json { render :index }
      format.csv { send_data build_csv, filename: "Orders-#{Date.today}.csv" }
    end
  end

  private

    def build_csv
      CSV.generate do |csv|
        csv << Order.csv_columns
        @orders.each { |order| csv << order.to_csv }
      end
    end

end