Downloading xls file from application

I am using spreadsheet gem to generate a xls file in my applications public folder and then using the send_file method to send the file. My code worked properly on my local machine in test as well as production mode. But when I deployed this code on heroku it showed the following error:

Permission denied - /app/public/history_23.x开发者_运维百科ls

Can anyone suggest the solution please.

Heroku has a read only file system, you CANNOT write to any where except the tmp and log directory -

http://devcenter.heroku.com/articles/read-only-filesystem