access json.jbuilder file in api

Missing template api/v2/subjects/fetch_subject, api/base/fetch_subject, application/fetch_subject with {:locale=>[:"zh-CN"], :formats=>[:html], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby, :coffee, :jbuilder]}.

新建的fetch_subject.json.jbuilder在接口访问不到， http://localhost:3000/api/v2/subjects/516?for_fetch=true
两种方法

(方法一) http://localhost:3000/api/v2/subjects/516.json?for_fetch=true
（方法2）

  if params[:for_fetch].present?  
    render :fetch_subject, formats: [:json] 
  else               
    render :show     
  end