EXT中COMBOBOX 的自动选中问题

new Ext.form.ComboBox({
                store:store,
                fieldLabel: '标签',
                hiddenName:'字段名',
                valueField:'字段名',
                displayField:'字段名',
                typeAhead: true,
                mode: 'local',
                triggerAction: 'all',
                emptyText:'提示',
                selectOnFocus:true,
                editable:true  }),