application.html.erb

  <%=form_for categories_path do |f| %>
    
      <%= f.select :parent_id, nested_set_options(Category, @category) {|i| "#{'-' * i.level} #{i.name}" } %>

      <%= select_tag 'parent_id', options_for_select(nested_set_options(Category) {|i| "#{'-' * i.level} #{i.name}" } ) %>
    
  <%end%>

categories.rb

class Category < ApplicationRecord

	acts_as_nested_set
	has_many :classifieds
end

categoriescontroller.rb

class CategoriesController < ApplicationController


	def index 

		@root_categories = Categories.roots

	end


	def show
		@categories = Categories.find_by_id(params[:id]).self_and_descendants
	end

end

2 spacing is easy to reading :)