polymorphic assocation with nested attributes and paperclip issue

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

<!-- language-all: lang-rb -->

    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 %>
    <div class="addl_attachments">
      <% 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 %>
      </div>
    </div>
    <div id="submit-button">
      <%=submit_tag "#{t('send')}", :class => 'send_button',
:disable_with => "#{t('please_wait')}" %>
    </div>

<% 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 %></div>
            <% else %>
                <div class="text-input-bg themed_text"
style="margin-top: 2px"><%= truncate(f.object.attachment_file_name,
:length => 15,:ommision => '...') %></div>
                <div class="remove_link"><%= link_to_remove_fields
image_tag('buttons/delete_2.png') ,f %></div>
            <% end %>
        <% end %>
      </div>
    </div>

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.