Filling treestore for Nested List from JS

Are there any tutorials on how to fill a treestore from Javascript?

What I want to do is load data from a WebSQL database and add it to the store to display it in a nested list. I've tried to use the store.add() and store.addData() functions, however with litte success.

It works if I try to load it from a local json file.

My store:

Code:

Ext.define('Muzic.store.Artists', {
    extend: 'Ext.data.TreeStore',

    config: {
        storeId: 'Artists',
        model: 'Muzic.model.Song',
        //WORKING, from http://ift.tt/1oZqJti:
        /*proxy: {
            type: 'ajax',
            url: '/data/catalogue.json',
            reader: {
                type: 'json',
                root: 'items'
            }
        }*/
       proxy: {
            type: 'memory',
            reader: {
                type: 'json',
                rootProperty: 'items'
            }
        }
    }
});

My Model:

Code:

Ext.define('Muzic.model.Song', {
    extend: 'Ext.data.Model',

    config: {
        fields: [
            {name: 'title', type: 'string'},
            {name: 'artist', type: 'string'},
            {name: 'filepath', type: 'string'},
            {name: 'alreadyHeard', type: 'boolean', defaultValue: false}
        ],
        validations: [{ type: 'presence', field: 'filepath'}]
    }
});

My nested list view:

Code:

Ext.define('Muzic.view.albums.Card', {

    extend: 'Ext.NestedList',
    xtype: 'nestedlist',

    config: {
        iconCls: 'album',
        title: 'Artists',
        store: 'Artists',
        displayField: 'title'   
    }
});

One try with addData:

Code:

    testAddData : function () {
        var arr = [];
        var obj = {items : [{
            title:'artist',
            filepath:'nothing',
                items:[{
                title: 'song',
                filepath:'somewhere',
                artist:'artist',
                leaf:'true'
                }],
            leaf:'false'
        }]};
        arr.push(obj);
        //console.log(JSON.stringify(obj));
        Ext.getStore('Artists').addData(arr);
        Ext.getStore('Artists').sync();
    },