Do you want this to be unique? Adding `unique: true` doesn't do anything. You need to add a unique index to the column. You should also validate this in the model.

- http://robots.thoughtbot.com/the-perils-of-uniqueness-validations
- https://tomafro.net/2009/08/using-indexes-in-rails-choosing-additional-indexes
- http://tomafro.net/2009/08/using-indexes-in-rails-index-your-associations