How to save a file on Heroku using a rake task?

Question

I have a rake task to update some fields in the mongo db using mongoid. I use CSV.open to create a file to store the error for anomalies which can't be updated programatically. When I run this locally it simply creates the CSV file in the root directory of the app. But when I run rake on Heroku it doesn't seem to save the file anywhere I can access it. Here's my code:

require 'csv'
CSV.open("errors.csv", "w") do |csv|
  csv << ["Email", "Jacket Size"]
  errors.each do |e|
    csv << e
  end
end

Answer 1

Heroku filesystem is read-only so your rake task cannot save this file in app root directory. With Heroku you can save only to /tmp and /log directories. You can create your CSV file in /tmp directory and after it's processed you have to save it at some external storage (like Amazon S3) or send via mail, whatever you think is better in your case.