How to add a child node in tree manually

[wqjh7809]

I have a tree with an ajax store like this:

        var tree = Ext.create('Ext.tree.Panel', {
            model: 'TreeModel',
            store: store,
            ...

with this store and model:

var store = Ext.create('Ext.data.TreeStore', {
            proxy: {
                type: 'ajax',
                url: '../api/tree'                
            },
            root: {
                text: 'Root',
                expanded: true
            },
            sorters: [{
                property: 'text',
                direction: 'ASC'
            }]
        });

Ext.define('TreeModel', {
            extend: 'Ext.data.Model',
            fields: [
                { name: 'id', type: 'string' },
                { name: 'text', type: 'string',},
                { name: 'entity', type: 'string' }
            ]
        });

By collapsing the nodes I sending a new request to my api-server. If the result is empty, I want to add a child node. For this I create a model instance, a node and trying to add them with appendChild, but it didn't work?

var testRecord = Ext.create('TreeModel', {
            id: 'testId',
            text: 'testText',
            entity: 'testEntity'
        });

In the 'afteritemexpand' listener at the tree I create and append the node:

var myNode = node.createNode(testRecord);
node.appendChild(myNode);

What's wrong in my code?