Rails Add rolify role via user form

I'm creating an application based on Devise, CanCan and Rolify to handle private resources. Devise registration has been disabled since I don't want allow users to registrer, while I'm creating an admin interface to add/edit users. Now I'm dealing with user role. I'd like to allow admin to create users and set their role using the same form. I don't know what kind of relation is using rolify because I'm pretty new to rails and Ruby and rolify added the `rolify` method instead of a more clear relation. I currently need and want to only have one role per user. So if someone edits the user changing the role I delete all `@user.roles` and then set the new one. I'm currently having it to work using a simple hack. I've added a select field to the form, using a collection of `Role`s to provide a selection of roles. Then inside the `create` and `update` methods I'm doing this: ``` role = params[:user][:role] params[:user].delete :role @user = User.new(params[:user]) respond_to do |format| if @user.save # update_attributes inside update @user.add_role role # ... end end ``` As you may see I'm not interacting with the `Role` model directly but using the provided `add_role` method. This also creates another issue, because I have to retrieve the current role to allow the update view to set the proper default value for the select (beside the most important fact that I don't know how to validate the presence of ONE role). This is going to be a too dirt solution, so I'd like to have some point of view, and maybe some suggestion on how to do this. I've spent all the day working on this :-( If you need some info please ask me. P.S. I'm watching a screenscast about nested resources but in the DB I may see that it's using a third table to keep track of `users_roles` and I'm not understanding how to do it. Thanks!!