怎么获得combobox的valueField值

var a;
       var dwField = new Ext.form.ComboBox({
            fieldLabel:'管理员',            
            mode: 'local',                           
            70,                                
            id:'user_name',                        
            name:'user_name',
            editable:false,
            allowBlank:true,
            typeAhead:true,
            triggerAction: 'all',
            selectOnFocus:true,
            store:Ext.basicdata.user,
            emptyText:'-请选择-',  
            hiddenName:'u_id',                                               //用户ID
            valueField: 'user_id',   
            displayField: 'user_name',    
     
            listeners:{        
                   select : function(combo, record,index)   
                   {    
                       jhField.clearValue(); //可以实现当队下拉值变更时，清空之前井号下拉选项中的值   
                       SegField.clearValue();
                       storejh.proxy= new Ext.data.HttpProxy({url: './DataPageHand.aspx?method=GetAirLine&user_id='+combo.value}); //根据队下拉选项的改变，动态取出对应的井
                       a=combo.value
                       storejh.load(); //加载航段下拉框的store
                       
                   }      
               }   
    });  

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alert(Ext.getCmp('user_name').getValue());
alert(Ext.getCmp('user_name').getRawValue());