paperclip and file upload issue

Question

I have a multi-step form inspired by this railcast that has 3 file upload images. However when I upload an image I receive an error can't dump File.

From other questions in SO I understand that saving a file uploads in the session doesn't work so I need ot use something like this:

   unless @post.valid?
      @post.assets.first.attachment.clear
      @post.assets.first.attachment.queued_for_write.clear
    end

This however doesn't seem to work. Should I look through the three file uploads and clear them every time? How can I avoid this error?

Here is my create function:

    session[:post_params].deep_merge!(params[:post]) if params[:post]
    session[:duration] = params[:post_duration] if params[:post_duration]


    @post = Post.new(session[:post_params])
    @post.current_step = session[:post_step]

   unless @post.valid?
      logger.info("attachment " +  @post.assets.first.attachment.inspect)
      @post.assets.first.attachment.clear
      @post.assets.first.attachment.queued_for_write.clear
    end


    if @post.valid?
      if params[:back_button]
        @post.previous_step
      elsif @post.last_step?
        if @post.all_valid?
          ...
          session[:post_step] = session[:post_params] = nil
          redirect_to @post and return
        end
      else
        @post.next_step
      end  

    session[:post_step] =  @post.current_step
    end

    if @post.new_record?
         render "new"
    end
  end

My asset model

class Asset < ActiveRecord::Base
  # attr_accessible :title, :body

 belongs_to :post
 attr_accessible :attachment
 has_attached_file :attachment, :styles => { :medium => "600x600>", :small => "200x200>", :thumb => "100x100>" }, 
 :default_url => "no_image_:style.jpg"
end

Answer 1

The problem is that you cannot store a File into the session object since it has to be Marshaled and that is not supported. As I see it you have three possible solutions:

1. The easy way out of this would be to add the image upload part at the last step of you wizard. That way you don't need to save the last part of information in the session but rather directly in your object. Just make sure that you skip the session save on the last step or you take out the file params before storing them to your session with something like

   params[:post].except(:assets)
   

2. The second solution is much trickier. What you can do is every time your controller tries to save a model you persist the attachments to the disk at a known place with a unique name. Now instead of storing the file to the session you will be storing the local file names that you just created (you still need something like the except from #1 to avoid the exception). Upon save you will basically restore the files from the server disk as specified in the session params. To keep things clean you need a cron job that would delete old files (e.g. any file saved a week ago). You can use whenever for that.

   Tip: If you choose the above solution and your attachments are a separate model, as it is seems, you can actually save the "temp" assets without an associated post_id. Then instead of file names you will handle unique database ids ;)

3. Lastly if your business logic (and your nerves) allow it, you can consider persisting your model in the database on every step, probably with a flag meaning that it is partially saved. That way you will not need the use of a session but you are going to have to deal with many conditionals (in your validations, in your scoping, in your associations ...) and of course you still need that cron job.