extjs 4 : get store record values

[skylinezpromise]

My json data looks like this :

({"success": "true", "message" : "OK","data":[{"id_metric":"1","name_filter":"doc filter","type_guicomp":"combobox"},{"id_metric":"1","name_filter":"severity","type_guicomp":"combobox"}]})

I'd like to be able to retrieve the "type_guicomp":"combobox" value for the "id_metric":"1", so the result would be : "type_guicomp":"combobox" and "type_guicomp":"combobox".

I do this because I need the value "combobox" to assign it to a variable, I've tried several things including :

myStore.load({
        scope: this,
        callback : function(record, operation, success) {
            console.log(record);
            console.log(record.data);
            }

And :

var index = Ext.StoreMgr.lookup("myStore").findExact('id_metric',1);
var rec = Ext.StoreMgr.lookup("myStore").getAt(index);
console.log(rec);

These solutions return undefined or null. So when I did this in the callback :

var i = myStore.getCount();
console.log(i);

It returned 0. But when I check the json output, it's not empty and there's actually data in it.
My store looks like this :

Ext.define('Metrics.store.MyStore', {
    extend: 'Ext.data.Store',
    model: 'Metrics.model.MyModel',
    autoLoad: true,
	idProperty: 'id_metric',
    proxy : {
		type : 'ajax',
		node : 'id_metric',
		api : {
		read : 'gui_comp_items.php'
		},
		reader: {
            type: 'json',
            successProperty: 'success',
            messageProperty: 'message',
            root: 'data'
        }}
});

My model :

Ext.define('Metrics.model.MyModel', {
    extend: 'Ext.data.Model',
    fields: [
	{name : 'id_metric', type : 'int'},
	{name : 'name_filter', type : 'string'},
	{name : 'type_guicomp', type : 'string'},
	{name : 'value', type : 'string'}]	
});

Pleaaase try to help me on this one, thank you!

[avcmc]

Please share the code of creating the store too, may be issue lies there, as here you have shared the code of defining it alone.