Passing Model ID in Upload

Greetings.

I am trying to create an image upload page for my site, and I am
having a little trouble figuring out how to set things up correctly.
I have a model for user's, each with a unique ID, and I would like to
have it so that when an image (i'm calling them artworks at the
moment) is uploaded it is saved to /somedir/<userid>/
<imageid>.<extension> The trouble is I can't figure out how to pass
the user ID info to rails so it will be saved in the image model. My
code looks something like this:

# artwork migration
  def self.up
    create_table :artworks do |t|
      t.column :title, :string
      t.column :media, :string
      t.column :description, :string
      t.column :user_id, :integer # foreign key to users table
      t.column :uploaded_at, :datetime
      t.column :file_path, :string
      t.column :file_name, :string
      t.column :content_type, :string
    end
  end

# upload template
<h1>Upload an artwork!</h1>

<% form_for(:artwork,
        :url => { :action => :upload },
        :html => { :multipart => true }) do |form| %>
  Title: <%= form.text_field("title", { :size => 20, :maxlength =>
100 }) %><br />
  Media: <%= form.text_field("media", { :size => 20, :maxlength =>
100 }) %><br />
  Description: <%= form.text_area("description", { :cols => 60, :rows
=> 40 })%><br />
  Select File to Upload: <%= form.file_field("uploaded_file") %><br />

  <%= submit_tag("Upload") %>
<% end %>

# controller
  def upload
    logger.debug("running upload")

    # Create an empty artwork to display in the form for GET/POST
    logger.debug("make a new Artwork")
    @artwork = Artwork.new(params[:artwork])

    # If something was submitted
    logger.debug("check for POST")
    if request.post?
      # If the save is successful, redisplay the page with a blank
artwork for next submit
      logger.debug("save to DB")
      if @artwork.save
        @artwork = Artwork.new
        flash[:notice] = "Artwork Uploaded Successfully"
      end
    end
  end

# Model for Artwork
class Artwork < ActiveRecord::Base
    belongs_to :user

    def uploaded_file=(incomming_file)
      logger.debug("running uploaded_file=")

      @temp_file = incomming_file
      @content_type = incomming_file.content_type.chomp
      @file_path = MEDIA_FOLDER + @user_id + '/' # **** error
here
      @file_name = @id + "." + @content_type
    end

    def after_save # called after a successful ActiveRecord save
      if @temp_file
        logger.debug("creating dir (if needed) '#{@file_path}'")
        File.makedirs @file_path
        logger.debug("saveing to file '#{@file_name}'")
        File.open("#{@file_path}#{file_name}", "wb+") do |f|
          f.write(@temp_file.read)
        end
      end
    end

    def after_destroy
      if File.exist?("#{@file_path}#{file_name}")
        File.delete("#{@file_path}#{file_name}")
      end
    end
end

When I run this code, I get an error at the line in the model above
that @user_id is nil and can't converted to a String. I know I need
to somehow get the current user's ID stored in the new artwork object,
but I can't figure out how. I have it stored in the session hash when
a user is logged in. I tried putting a hidden filed in the template
like this:

<%= form.hidden_field("user_id", { :value => "#{@current_user_id}" })
%>

Where @current_user_id is grabs the id from the session, but same
error.

I tried assigning the value in the controller just before calling
@artwork = Artwork.new(params[:artwork]), but this seems to overwrite
that value.

I tried accessing the session from within the model, but it looks like
it's not in scope for that code. Any help would be appreciated.

Thanks,
Matthew