Rails - Mar 2006 - using ''onchange'' option in select from a partial to update a

I''m new to Rails and don''t totally understand some of the
control
behavior of ActionController. Here''s my problem: I have a _header.rhtml
partial view that is supposed to be the header (naturally) on a 
main.rhtml view (i.e., main.rhtml calls the header partial via the 
render method). In the header view is a select field that I''d like to 
have refresh the main.rhtml view (altering the view based on the 
selected value) but I''m having a hard time understanding the mechanics 
of the observe_field method. Here''s my _header partial:

<%= start_form_tag :action => ''main'' %>
<div="tspan">
<%= select(:timespan, :span, ["1 week", "1 month",
"3 months", "6
months", "1 year"],  :onchange => ''main'')
%>
</div>
<%= end_form_tag %>
<%= observe_field(
	:timespan,
        :frequency => 0.25,
	:update => ''clientresults'',
        :url => {:action =>:main},
	:with => ":timespan") %>

So I''m looking to set an instance variable in the main method in my 
controller with the value of the select, and then have the main method 
called again with that instance variable set. I know this code is broken 
but I don''t know how. Can anyone help?

Thanks

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