I'm a new user of extjs, and I've got an issue.

I have this Json :
{"total":"5", "corporates":[{"cop_id":"2","cop_name":"burode"},{"cop_id":"3","cop_name":"tag"},{"cop_id":"1","cop_name":"tagattitude"},{"cop_id":"4","cop_name":"test"},{"cop_id":"5","cop_name":"test2"}]}
I'm trying to display this in this comboBox using a Ext.data.Store and a Ext.data.JsonReader like this :
        var ds = new Ext.data.Store({
		reader: new Ext.data.JsonReader({
			root: 'corporates'
			, totalProperty: 'total'
		}, [{name:'cop_id'}, {name:'cop_name'}])
	var comboBox = new Ext.form.ComboBox({
		store: ds,
		mode: 'local',
		valueField: 'cop_id',
		displayField: 'cop_name',
		applyTo: 'corporateList'
corporateList is here in my php file :
PHP Code:
input type="text" id="corporateList" size="20"/>
The matter is that nothing is inside my ComboBox....

Is there someone to help me on this ?

Thanks a lot.