参考示例:ComboBox:联动选择
                      一:创建两个ComboBox  
<span>部门</span><br />
<input id="deptCombo" class="mini-combobox" style="width:150px;" textField="name" valueField="id" 
    onvaluechanged="onDeptChanged" url="../data/DataService.aspx?method=GetDepartments"
    showNullItem="true"
        />         
<br /><br />
<span>职位</span><br />
<input id="positionCombo" class="mini-combobox" style="width:150px;" textField="name" valueField="id" />  
            二:valuechanged加载第二个ComboBox
function onDeptChanged(e) {
    var id = deptCombo.getValue();
    positionCombo.setValue("");
    var url = "../data/DataService.aspx?method=GetPositionsByDepartmenId&id=" + id
    positionCombo.setUrl(url);            
}