Read through the commits to understand how this application has been generated and how Sequel has been later introduced to replace AR for the User model.

Unfortunately Devise assumes too many things instead of relying on orm_adapter API.

That forces us to use some Sequel plugins like :dirty (for helping with email_changed? expected method from Devise when registering a new user through their common interface) and :validation_class_methods.

You'll probably need to implement a few other methods for using some of the Devise modules.

Anyway, this may help you getting the basics working so that you can go forward from there.

If you want to remove the active_record gem dependency but want to keep using AR migrations you may be interested in projects like standalone_migrations or Active Record Migrations.