EXTjs treegrid - correct model - json

Question

A very simple example, but it does not work. List of Working Groups looped. What am I doing wrong? How to correct write a model for this json. Docs
http://docs.sencha.com/extjs/4.0.7/#!/api/Ext.data.reader.Reader
Below is a screenshot result.
https://ps.vk.me/c537518/u155966612/doc/2ee8ec3e7718/1234.png
JS code.
Ext.define('WorckGroup', {
extend: 'Ext.data.Model',
fields: [
{ name: 'text', type: 'string', mapping: 'WorckGroup' },
],
hasMany: { name: 'children', model: 'UserReport', associationKey: 'UserReport' }
});
Ext.define('UserReport', {
extend: 'Ext.data.Model',
fields: [
{ name: 'text', type: 'string', mapping: 'Specialist' },
],
belongsTo: 'WorckGroup'
});
var store = new Ext.data.TreeStore({
model: 'WorckGroup',
proxy: {
type: 'ajax',
url: '/omnireports/ajaxgrid'
},
autoLoad: true,
folderSort: true
});
Ext.define('basegrid', {
extend: 'Ext.tree.Panel',
xtype: 'tree-grid',
title: 'Core Team Projects',
height: 300,
useArrows: true,
rootVisible: false,
multiSelect: true,
singleExpand: false,
initComponent: function () {
this.width = 500;
Ext.apply(this, {
store: store,
columns: [{
xtype: 'treecolumn',
text: 'Task',
flex: 2,
sortable: true,
dataIndex: 'text'
},]
});
this.callParent();
}
});
json
[{"WorckGroup":"Группа поддержки 3D",
"UserReport":[
{"Specialist":"Вагин Александр Константинович","SCallCount":64,"AverageDuration":0.1136067,"leaf":true},
{"Specialist":"Кистерева Татьяна Алексеевна","SCallCount":171,"AverageDuration":0.1058641,"leaf":true},
{"Specialist":"Лысенко Алексей Валентинович","SCallCount":71,"AverageDuration":0.4269940,"leaf":true}]},
{"WorckGroup":"Группа поддержки сервиса КСУП",
"UserReport":[
{"Specialist":"Болгов Руслан Евгеньевич","SCallCount":62,"AverageDuration":0.1093749,"leaf":true},
{"Specialist":"Костин Алексей Алексеевич","SCallCount":17,"AverageDuration":0.1125816,"leaf":true},
{"Specialist":"Мироненко Анна Владимировна","SCallCount":172,"AverageDuration":0.0632347,"leaf":true},
{"Specialist":"Ползиков Мирослав Александрович","SCallCount":315,"AverageDuration":0.0945766,"leaf":true},
{"Specialist":"Тахтенков Николай Владимирович","SCallCount":7,"AverageDuration":0.2342261,"leaf":true}]}