streaming_csv_controller.rb

class StreamingCsvController < ApplicationController
  def test
    respond_to do |format|
      format.csv {render_csv}
    end
  end
  private

  def render_csv
    set_file_headers
    set_streaming_headers

    response.status = 200

    #setting the body to an enumerator, rails will iterate this enumerator
    self.response_body = csv_lines
  end


  def set_file_headers
    file_name = "transactions.csv"
    headers["Content-Type"] = "text/csv"
    headers["Content-disposition"] = "attachment; filename=\"#{file_name}\""
  end


  def set_streaming_headers
    #nginx doc: Setting this to "no" will allow unbuffered responses suitable for Comet and HTTP streaming applications
    headers['X-Accel-Buffering'] = 'no'

    headers["Cache-Control"] ||= "no-cache"
    headers.delete("Content-Length")
  end

  def csv_lines

    Enumerator.new do |y|
      y << "#{["i", "rand"].join(",")}\n"

      i = 1
      Datum.find_each do |d|
        y << "#{d.i},#{d.value}\n"
        puts "line #{d.i} added"
        sleep 0.01
      end
    end

  end
end