controller.rb

  def show
    topic_ids = if (read = ForumTopicRead.find_by(user_id: current_user.id, forum_topic_id: 0))
      ForumTopic.unscoped.joins("LEFT JOIN forum_topic_reads ON forum_topic_reads.forum_topic_id = forum_topics.id AND forum_topic_reads.user_id = #{current_user.id}").where(forum_id: params[:id]).where('forum_topics.last_post_at > ?', read.updated_at).group('forum_topics.id').having('count(forum_topic_reads.id) < 1').pluck(:id)
    else
      ForumTopic.unscoped.joins("LEFT JOIN forum_topic_reads ON forum_topic_reads.forum_topic_id = forum_topics.id AND forum_topic_reads.user_id = #{current_user.id}").where(forum_id: params[:id]).group('forum_topics.id').having('count(forum_topic_reads.id) < 1').pluck(:id)
    end
    render json: topic_ids
  end