Synchronize country and city selection in a form

city_partial.js.erb

<% if !@cities.blank? %>
$('#cityfield').html("");
$('#cityfield').html("<%=j collection_select(:resource,:CITY_ID,Country.find(1).cities,:id,:NAME, {:prompt =>'Select a city'})%>");
<%else%>
$('#cityfield').html("");
$('#cityfield').html("<%=j text_field :CITY_ID, :disabled%>");
<%end%>

controller.rb

  def edit
    @title = "Edit resource info"
    @station = Resource.find(params[:id])

    #protect the ui from null values
    if !@station.CITY_ID.blank? && @station.CITY_ID>0
      @station_country_name = Country.find(City.find(@station.CITY_ID).COUNTRY_ID).NAME
      @station_city_name = City.find(@station.CITY_ID)
    else
      @station_country_name = ""
      @station_city_name = ""
    end
  end
...

def city_partial
  if !params[:country_id].blank? && params[:country_id].to_i>0
    @cities = Country.find(params[:country_id]).cities 
  else
    @cities = nil
  end
end

page.html.erb

...

<% f.collection_select(:country,Country.all,:id,:NAME, 
    :include_blank => (true if @station_country_name.blank?), 
    :selected => (@station_country_name unless @station_country_name.blank?) )%>
<span class="help-block">Country</span>

...

<% f.collection_select(:CITY_ID, ( ([@station_city_name] unless @station_city_name.blank?) || []),:id, :NAME, :selected => @station_city_name ) %>
<span class="help-block">City</span>

...

<script type="text/javascript">
    $("#resource_country").change(function(){
	    var country_id = $('select#resource_country :selected').val();
        if(country_id =="") country_id="0";
        $.get('/users/resource/city_partial/'+country_id);
    });
</script>
<% end %> //of form