Rails 5.2 with master.key – Heroku deployment

You should set the environment variable RAILS_MASTER_KEY, either on your heroku web dashboard, or using console:

$ heroku config:set RAILS_MASTER_KEY=<your-master-key>

Example:

$ heroku config:set RAILS_MASTER_KEY=123456789

(The < and > are placeholders)

Rails will detect this variable and use it as your master key (instead of looking for it in master.key file).