Monkey-patch mongoid-slug to allow slug = _id

mongoid_slug_unique_slug.rb

module Mongoid
  module Slug
    class UniqueSlug

      alias_method :_find_unique, :find_unique

      # by default Mongoid::Slug returns id-1 if you try to get a slug from the model's ID
      def find_unique attempt = nil
        if ! attempt
          candidate = url_builder.call(model)
          return candidate if candidate == model.id.to_s
        end
        _find_unique attempt
      end
    end
  end
end

See a discussion on allowing IDs as slug in mongoid/mongoid-slug#91.