application_controller.rb

# In the controller#action
respond_to do |format|
  format.csv { render_streaming_csv(generate_csv(column_names, records)) }
end
 
 
def render_streaming_csv(csv_enumerator)
  # Delete this header so that Rack knows to stream the content.
  headers.delete("Content-Length")
  # Do not cache results from this action.
  headers["Cache-Control"] = "no-cache"
  # Let the browser know that this file is a CSV.
  headers['Content-Type'] = 'text/csv'
  # Do not buffer the result when using proxy servers.
  headers['X-Accel-Buffering'] = 'no'
  # Set the filename
  headers['Content-Disposition'] = "attachment; filename=\"report.csv\"" 
  # Set the response body as the enumerator
  self.response_body = csv_enumerator
end