gistfile1.rb

#EN LA VISTA 
<%= hidden_field_tag(:old_employee_id, @old_employee_id) %>	
<%= f.select("new_employee_id", @employees.collect{|e| [ e.name + " " + e.last_name, e.id ] }, {:include_blank => true, :selected => @old_employee_id}) %>
...

#EN EL CONTROLADOR

# en edit

@card = Card.find(params[:id])
...
...
active_assignment =  @card.active_assignment
@old_employee_id = active_assignment.employee_id if !active_assignment.nil?

# en update
if @card.update_attributes(params[:card])
		if params[:card][:new_employee_id] != params[:old_employee_id]
			@card.reassign(params[:card][:new_employee_id])
		end
        format.html { redirect_to(@card, :notice => 'Card was successfully updated.') }
        format.xml  { head :ok }
      else
...

#NUEVAS COSAS EN EL MODELO

class Card < ActiveRecord::Base
 
def active_assignment()
	self.card_assignments.all(:conditions=>["active_since < '#{DateTime.now.utc}' and '#{DateTime.now.utc}' < active_until"]).first
 end