github 中有个 gem：china_city(https://github.com/saberma/china_city) 怎样在 activeadmin 下实现这个功能？

form :url => "/fieldwork/interviewers/profile"  do |f|
  f.inputs "Detail" do
      f.input :birthdate, :as => :datepicker
      f.input :gender
      f.input :ethnicity
      f.input :origin
      f.input :local_languages
      f.input :province
      f.input :city
      f.input :aera
    end
  f.actions
end

怎样让

f.input :province
f.input :city
f.input :aera

实现级联？