Rails: ActiveAdmin で has_many なフォーム ( nested resouces ) を作るための最小構成 ref: http://qiita.com/Yinaura/items/e4cad1b59afe08b7de11

book.rb

ActiveAdmin.register Book do

  permit_params :title, images_attributes: [:kind]

  form do |f|
    f.inputs do
      f.input :title
    end

    f.inputs do
      f.has_many :images do |t|
        t.input :kind
      end
    end

    f.actions
  end
end

image.rb

class Image < ActiveRecord::Base
end