Symfony 1.4: How I can retrieve the selected value with AJAX's function in select dependent?

In my database I have two related fields. The second field depends on the value selected in the first. The relations are:

The function I use in the form of table "conflictos_1" is:

<!--Aquí el javascript para select dependientes-->
<script type="text/javascript">
$(document).ready(function()
{
    $("#conflictos1_id_sector_actividad").change(function()
    {
        var id_sub = $(this).val();
        if(id_sub != '')
        {
            $.ajax
            ({
                type: "POST",
                url: '<?php echo url_for('conflictos/subsector'); ?>'+ '?id=' + id_sub,
                cache: false,
                data: "id_sub="+ id_sub,
                success: function(data)
                {
                    $("#conflictos1_id_subsector_actividad").html(data); // but it does not select the value of dropdown list.
                }
            });
        }
        else
        {
            $("#conflictos1_id_subsector_actividad").html("<option value=''>-- No se ha seleccionado subsector --</option>");
        }
        return false;
    });
});
</script>

When I add a new record, everything works fine. But when I edit a record, the select dependent, does not show "selected" value. In edit mode, when I look at the field "id_subsector_actividad", the selected value should be, for example, <option value="37 " selected="selected">: This is what I see on my form when I inspect the element created with AJAX's function:

<select name="conflictos1[id_subsector_actividad]" id="conflictos1_id_subsector_actividad">
   <option value="29 ">14.1 Meretrices</option>        
   <option value="30 ">Preparación de alimentos y comedor</option>        
   <option value="31 ">Seguridad</option>        
   <option value="37 ">redes sanitarias</option>        
</select>

This is what I WANT to see:

<select name="conflictos1[id_subsector_actividad]" id="conflictos1_id_subsector_actividad">
       <option value="29 ">14.1 Meretrices</option>        
       <option value="30 ">Preparación de alimentos y comedor</option>        
       <option value="31 ">Seguridad</option>        
       <option value="37 "  selected="selected">redes sanitarias</option>        
    </select>

I use this function to filter records in the table "Subsector_actividad_ta8"(I work with Symfony 1.4 and Doctrine):

    public function executeSubsector() 
    { $id_sub = $_POST['id_sub']; 
$this->subsec= Doctrine_Core::getTable('SubsectorActividadTa8') ->createQuery('a') 
    ->where('a.id_sector = ?', $id_sub) 
    ->execute(); 
    } 

My question is: What should I change in the AJAX's function, to display the "selected" value in the second field, when I am editing an existing record?

Here the solution I encountered: I've separated the problem in two.

Case 1: New Record

I use the same JQuery function shown above in the question. With the associated function, also shown above. This case was not part of my problem

Case 2: Edit an existing record (here was my problem)

I added for 'id_subsector_actividad' in the widget the property 'table_method'=>'Subsector' associated with the function public function Subsector() (see below code).

In the partial _form.php of conflictos1, I wrote the following code for the field id_subsector_actividad:

<?php if (!$form->getObject()->isNew()): ?>       
         <?php echo $form['id_subsector_actividad'] ?>    
    <?php endif; ?>                 
 <?php if ($form->getObject()->isNew()): ?>   
             <select name="conflictos1[id_subsector_actividad]" id="conflictos1_id_subsector_actividad">

              <option value="" selected="selected">Seleccione sub-sector</option>
  <?php endif; ?>  

In the action.class.php in the conflictos1, I modified the function executeEdit, with the addition of the variable $elsector:

public function executeEdit(sfWebRequest $request)
  {
 global $elsector;
    $this->forward404Unless($conflictos1 = Doctrine_Core::getTable('Conflictos1')->find(array($request->getParameter('id'))), sprintf('Object conflictos1 does not exist (%s).', $request->getParameter('id')));

    $elsector= $conflictos1->getIdSectorActividad(); 


    $this->form = new Conflictos1Form($conflictos1);


  }

I use $elsector in the following function created in SubsectorActividadTa8Table.class.php

public function Subsector()

    {
         global $elsector;


         if (!is_null($elsector)){
       $id_sub=$elsector;
       $query= Doctrine_Query::create()
            ->select('a.id')
            ->from('SubsectorActividadTa8 a')
         ->innerJoin('a.Conflictos1 c')

           ->where('a.id_sector = ?', $id_sub); 
               return $query->execute();  
    }
    }

Thus, Symfony displayed on the form, the value of field named IdSubsectorActividad, not previously exhibited.

In other words, now when I'm editing a record in the table conflictos1 the field named IdSubsectorActividad shows the correct value, whereas before the form does not show any value.

Now everything works fine!.