ext二级联动兑现

ext二级联动实现
  今天用ext实现二级联动，原来以为很简单，但是试了半天，第二个下拉列表就是不能显示
后来发现后台向前台传json值用了对象，显示就出了问题，费了半天劲才搞出来，

 

certifcateTypeStore = new Ext.data.JsonStore({
				    url:'../getCertificateTypeListByArray',				   
				    root:'certificateTypeList',
					fields:['certificateTypeId','certificateTypeName']					
			     });
			 qytg_certificateTypeComb=new Ext.form.ComboBox({
					margins:'0 0 0 0',					 
					fieldLabel: '证书类型',
					displayField:'certificateTypeName',
					valueField:'certificateTypeId',
					allowBlank:true, 
					height: 25,
		            anchor: '95%',
					editable : false,
					mode:'local',
					store:certifcateTypeStore,
					triggerAction : 'all'		
				 });
			 certifcateLevelStore= new Ext.data.JsonStore({				  
					fields:['certificateLevel']					
			     });	 

			 var qytg_certificateLevelComb=new Ext.form.ComboBox({
					margins:'0 0 0 0',	
					//disabled:true,
					 //forceSelection: true,				
					fieldLabel: '证书等级',
					displayField:'certificateLevel',
					valueField:'certificateLevel',
					allowBlank:true, 
					anchor: '95%',
					editable : false,
					mode:'local',
					store:certifcateLevelStore,
					triggerAction : 'all'		
				 });
			 qytg_certificateTypeComb.on('select', function (combo, record,index) {  
				 qytg_certificateLevelComb.clearValue();                 
                 Ext.Ajax.request({  
                 url :'../getCertificateLevelList',  
                 mthod : 'post',   
                 params:{  
                	 certificateID:qytg_certificateTypeComb.getValue()  
                 },  
                 success : function(response) {  
                          var result = Ext.util.JSON.decode(response.responseText);                       
                          qytg_certificateLevelComb.getStore().loadData(result.levellist);  
                      }  
              })  ;
              } );