cascading combobox with this simplest example
<p>from serverside I'm returning data in following structure
 results.houses 
 where each house and <code>Id</code> and <code>Name</code> and Doors collection. 
 Each <code>Door</code> has <code>Id</code> and <code>Name</code>.</p>

<pre><code> $.each(results.houses, function (index, value) {
    $('#HouseId').append('&lt;option value="' + value.Id + '"&gt;' + value.Name + '&lt;/option&gt;');   
    var selectedHouse = results.SelectedHouse[0].Id;  

    $('#HouseId').val(selectedHouse);
    $('#HouseId').combobox('refresh');
 }
</code></pre>

<p>How can I populate Doors combobox with <code>#DoorId</code> 
if I already know selected house from above variable <code>selectedHouse</code>.</p>

<p><strong>Update:</strong></p>

<p>json data is returned in following format from the server side controller</p>

<pre><code>var _houses = HousesRepository.All().Select(x =&gt; new
{
    Id = x.Id, Name = x.Name,
    Doors = x.Doors.Select(a =&gt; new { Id = a.Id, Name = a.Name, IsDefault = a.IsDefault })
});

var data = new 
{
    selectedHouse = new[] { new { Id = 1, Name = "House 100 "} },
    houses = _houses    
}    
return Json(data);
</code></pre>