def topic_nested_list(topics_list)
       get_nested_list(topics_list, topics_list.first)
      end

      def get_nested_list(topics, parent)

       ul_contents = ""
       ul_contents << "<ul>"
       childs = get_topic_childs(topics,parent.id)

       if childs.blank?
        ul_contents << "<li>" << parent.title << "</li>"
       else
        for child in topics
         ul_contents << get_nested_list(topics, child)
        end
       end
       ul_contents << "</ul>"
      end

      def get_topic_childs(topic_list, id)

       childs = []
       topic_list.each do |topic|
        if topic.parent_id == id
         childs.push(topic)
        end
       end

        return childs
       end

def process_topics topics_list
  topics_list.each do |t|
    # do something
    process_topics children_of_topic(topic_list, t)
    # or do something
  end
end

def children_of_topic(topic_list, topic)
  topic_list.select(|t| t.id == topic.parent_id)
end

get_nested_list(topics_list, topics_list.first)