Polymorphic assocation with nested attributes and paperclip issue

Rails
1

I have a Student model and an AddlDocument model (based on paperclip).

class Student < ActiveRecord::Base
  has_many,:addl_documents, :as => 'receiver', :dependent =>

:destroy
accepts_nested_attributes_for, :addl_documents, :allow_destroy =>
true
end

class AddlDocument < ActiveRecord::Base
  has_attached_file :doc_attachment,
                    :url =>

“/system/:class/:attachment/:id/:style/:basename.:extension”,
:path =>
“:rails_root/public/system/:class/:attachment/:id/:style/:basename.:extension”
belongs_to :receiver, :polymorphic=>true
end

I want to add multiple attachments for a particular student. Here is the
form :

<% form_for (@asset,:url=>{:controller=>'addl_documents',

:action=>‘save_a’}, :html=>{:multipart=>true}) do |f| %>
<%= f.hidden_field :id, :value=>@document.id %>
<%= label_tag :document_type %>
<%= select_tag ‘document_type’,
options_for_select(@doc_types.collect{ |doc| [doc.document_type, doc.id]
}) %>
<%= label_tag :description %>
<%= text_area_tag ‘description’, nil,:cols => 25, :rows => 4 %>
<%= label_tag :notes %>
<%= text_area_tag ‘notes’, nil,:cols => 25, :rows => 4 %>


<% f.fields_for :addl_documents do |d| %>
<%= render “addl_document_fields”, :f=>d %>
<% end %> 
  <div class="add_addl_attachment">
    <%= link_to_add_addl_attachment "#{image_tag "buttons/add_2.png"

} Add", f, :addl_documents %>




<%=submit_tag “#{t(‘send’)}”, :class => ‘send_button’,
:disable_with => “#{t(‘please_wait’)}” %>

<% end %>

Then I go to this forms page, I fill in the fields, and click on the
submit button. The server try to renders the page, but it doesn’t work
because I have the following error :

undefined method `klass' for nil:NilClass

My partial page(_addl_document_fields) contains the following

<div class="fields">
  <div class="label-field-pair-attachment">
    <label for="student_image_file"><%= t('attachment') %></label>
    <% if @asset.new_record? %>
        <div class="text-input-bg">

          <%= f.file_field :doc_attachment, :size=>12 %></div>
    <% else %>
        <% if f.object.attachment_file_name.nil? %>
            <div class="text-input-bg"><%= f.file_field

:doc_attachment, :size=>12 %>
<% else %>

<%= truncate(f.object.attachment_file_name,
:length => 15,:ommision => ‘…’) %>

<%= link_to_remove_fields
image_tag(‘buttons/delete_2.png’) ,f %>

<% end %>
<% end %>

My controller code are as follows:

def new
   @doc_types = DocumentSetting.find(:all, :order => 'document_type

asc’,:conditions => “status = true”)
@document = Student.find(params[:id])
@asset = @document.addl_documents.build
end

  def save_a
    if request.post?
      @document = Student.find(params[:addl_document][:id])
      @asset =

AddlDocument.new(params[:addl_document][:addl_documents])
@asset.save
end
end

I am not sure whether I have set up the associations correctly and
written the controller code accordingly as I am just learning Rails
associations and new to ROR. Please help.