Faster CSV downloads using Enumerator

faster_csv_downloads_using_enumerator.rb

Faster CSV downloads using Enumerator 
https://medium.com/reflektive-engineering/faster-csv-downloads-using-enumerator-7e9b94b870d3

We do not have to download the full movie first to start watching it thanks to the concept of ‘media streaming’. In simple terms, we can download chunks of the movie in sequence while we’re watching the downloaded chunks.
Similarly, we can stream the CSV to the customer, rather than make them wait for the complete CSV to be generated.

This is possible because Rack, the middleware that Rails uses, requires the response object to support the #each method. Rack uses #each to send the response back in chunks.

In the current scenario, the CSV file that’s generated (Refer to Step#2 in the process) is a String. String does not support #each, and Rack is forced to send all of it in one shot, after it’s ready.

So, Rack requires the response object to support #each and ruby’s Enumerator fit this requirement.


def generate_csv(column_names, records)
  Enumerator.new do |csv|
    csv << column_names.to_csv # add headers to the CSV
    records.each do |record|
      csv << record.attributes.values_at(*column_names).to_csv
    end
  end
end


# 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