wanting values stored and linked drop down list

I have 3 models...
facility - has_many :placements, has_many :client_slots
placement - belongs_to :facility, has_one :client_slots
client_slot - belongs_to :placements, has_one :facility

My problem is that I want a select list of client_slots based upon
facility_id which I get from the below code, but if the placement has a
'client_slot' already, it doesn't indicate it on the form. I want it to
show the existing value and to offer choices based upon the facility_id

on a placement form...

  <%= options = [['Select a Facility', '']] + @facility.sort { |a,b|
     a.name <=> b.name }.collect {
       >fac> [fac.name, fac.id] }
       select 'placement', 'facility_id', options %>

  <%= select_tag 'placement', 'client_slot_id', { :disabled =>
     'disabled' } %></span>
  <%= observe_field(
      :update => :client_slot_id,
      :url => { :controller => 'placements',
      :action => :lookup_client_slot },
      :with => "'facility_id='+escape(value)") %>

and in controller
  def lookup_client_slot
    @client_slots = ClientSlot.find(:all,
      :conditions => ["facility_id = ?", params[:facility_id]]
    ).collect {|fac| [fac.name, fac.id]}
      render :inline => "<%= select 'placement', 'client_slot_id',
        @client_slots %>"