Q:

how to restrict active admin to admin users only?

I have a user model with rolify gem. In my app I am using active_admin as admin interface.
Have can I restrict active_admin to admin users only?

A:

Try putting your rolify role check in the same place as mentioned in the Active Admin documentation's example for getting access to the current user:
class OnlyAdmins < ActiveAdmin::AuthorizationAdapter
  def authorized?(action, subject = nil)
    # Do rolify check here
    user.has_role? :admin
  end
end

