ruby-on-rails - 更新属性不起作用

Question

我有两列：

background_image_id (which is a relation)
background_image (which is a copy of that image, with various versions)

在我的before_update方法中，我必须将 Carrierwave 文件从Medium.find(background_image_id)列background_image中复制。

而我做不到。NIL 被存储到数据库中。不知道为什么。

---

控制器：

def update
  if params[:profile].blank? || params[:profile][:background_image_id].blank?
    flash[:alert] = 'Please select image'
    redirect_to edit_candidate_background_image_path
  else
    resource.background_image_id = params[:profile][:background_image_id]

    if resource.save
      flash[:notice] = 'Profile was updated'
     else
      flash[:alert] = "Can't update profile"
    end

    redirect_to edit_candidate_background_image_path
  end
end

模型：

before_update :copy_background_image, if: :background_image_id_changed?  
...

def copy_background_image
  medium = Medium.find(background_image_id)
   if medium.present?
    self.background_image = medium.image.file
    puts self.inspect # outputs a model where background_image is nil
    puts self.background_image.inspect # outputs an uploader with correct image PRESENT
  end
end