Ruby on rails Activeadmin导入或上载图像

我需要上传一个图像到我的activeadmin没有使用任何宝石。这是我的密码。 app/admin/article_month.rb

ActiveAdmin.register ArticleMonth do
menu label: "Month", parent: "News Digest"



    permit_params :month, :year, :image, :published_at

  index do
    selectable_column
    column :month
    column :year
    column :image
    column :published_at
    actions
  end
 form multipart: true do |f|
    f.inputs do 
        f.input :month, :as => :select, :collection => ["January", "February", 
            "March", "April", "May", "June", "July", "August", "September",
            "October", "November", "December"]
        f.input :year
        f.input :image, :as => :file
        f.input :published_at, :as => :date_picker
    end
    f.actions
 end


end

但它会返回如下错误：

NoMethodError in Admin::ArticleMonthsController#create
undefined method `name' for nil:NilClass

我不知道“名字”是从哪里来的。 有什么办法解决我的问题吗？ 谢谢。

此代码适用于我：）

 form multipart: true do |f|
    f.inputs do 
        f.input :month, :as => :select, :collection => ["January", "February", 
            "March", "April", "May", "June", "July", "August", "September",
            "October", "November", "December"]
        f.input :year
        f.input :image, input_html: { type: :file, class: "default"}, wrapper: false
        f.input :published_at, :as => :date_picker
    end
    f.actions
 end