I don't think there's a way to do it without Javascript.
I've used the one at http://www.xaprb.com/blog/2005/09/29/javascript-combo-box/
And then used this in the form- <td><label for="invoice_project_name">Project name</label><br/> <%= @projects=Invoice.find_by_sql(["select distinct project_name from invoices where client = ? order by project_name",@invoice['client']]).map {|u| [u.project_name, u.project_name]} @projects=@projects.unshift(@invoice['project_name']) select(:invoice, :project_name, @projects, { }, {:onkeypress=>"edit(event);", :onblur=>"this.editing=false;"})%></td>
