jbuilder 的 extract!

    #json.id subject.id
    #json.title subject.title
    #json.sub_title subject.sub_title
    #json.categories subject.categories
    #json.description subject.description
    #json.status subject.status
    #json.type subject.type
    #json.use_default_image subject.use_default_image
    #json.has_ranking_list subject.has_ranking_list

    替换为：
    json.extract! subject, :id, :title, :sub_title, :categories, :description, :status,
        :type, :use_default_image, :has_ranking_list