how to add html id to rails form_tag

For <element>_tag you specify HTML attributes directly, as follows:

<% form_tag session_path, :id => 'elm_id', :class => 'elm_class',
                          :style => 'elm_style' do -%>
   ...
<% end -%>

It is for <element>_remote_tag constructs that you need to move the HTML attributes inside a :html map:

<% form_tag form_remote_tag :url => session_path, :html => {
                            :id => 'elm_id', :class => 'elm_class',
                            :style => 'elm_style' } do -%>
   ...
<% end -%>

This code

form_tag(:controller => "people", :action => "search", :method => "get", :class => "nifty_form")

give as result:

<form accept-charset="UTF-8" action="/people/search?method=get&class=nifty_form" method="post">

But, if you use this structure

form_tag({:controller => "people", :action => "search"}, :method => "get", :class => "nifty_form")

you will get as result

<form accept-charset="UTF-8" action="/people/search" method="get" class="nifty_form">

and it's exactly what you want

<% form_tag 'session_path', :id => 'asdf' do -%>      
<% end -%>

Generates

   <form action="session_path" id="asdf" method="post"><div style="margin:0;padding:0"><input name="authenticity_token" type="hidden" value="ed5c62c836d9ba47bb6f74b60e70280924f49b06" /></div>
   </form>