Topic: Passing static variable from a form

I have a form that I use to allow names in the DB to be updated...this works.
I added a new field in my DB called "updated_by" because I want to know the username of the person the edits the record.

I want to be able to now pass @sessionuser.login which contains the username behind the scenes from the form to my controller without having to ask for it.

I thought I would add it in my edit.rhtml view on the first line as follows but this doesn't work:

<%= start_form_tag :action => 'update', :id => @link, :updated_by => @sessionuser.login %>
<% if @sessionuser.login == "administrator" %>
  <%= render :partial => 'formadmin' %>
<% else %>
  <%= render :partial => 'form' %>
<% end %>
  <%= submit_tag 'Edit' %>
<%= end_form_tag %>

I also tried putting it in my controller under "update" as follows but this doesn't work either:
def update
    @link.updated_by =  session[:user]
    @link = Link.find(params[:id])
    if @link.update_attributes(params[:link])
      flash[:notice] = 'Record was successfully updated'
      redirect_to :action => 'indexadmin', :id => @listall
    else
      render :action => 'edit'
    end

Any help would really be appreciated....thanks.
jackster