如何在Google Charts中使用JSON？

I want use JSON in Google Charts. This is my JSON Result:

[
    {
        "Ders": "Coğrafya",
        "SoruSayisi": "17"
    },
    {
        "Ders": "deneme",
        "SoruSayisi": "7"
    },
    {
        "Ders": "FEN BİLGİSİ",
        "SoruSayisi": "8"
    },
    {
        "Ders": "Geometri",
        "SoruSayisi": "5"
    }


]

My PHP Code:

public function getLessonCount()
            {
                $result = $this->getModelsManager()->createBuilder()
                    ->addFrom("\App\Models\Content\Dersler", "d")
                    ->columns(array("Ders"=>"d.dersAdi","SoruSayisi"=>"count(q.dersId)"))
                    ->innerJoin("\App\Models\CmsQuestion",'q.dersId=d.dersId', "q")
                    ->groupBy("d.dersAdi")
                    ->getQuery()->execute();

                return $result;
            }

JavaScript code in AngularJS:

//google visualization loader
    google.load('visualization', '1', {'packages':['corechart']});
        google.setOnLoadCallback(getDatas());
        function drawChart() {
            var data = new google.visualization.DataTable([$scope.datas]);
            var options = {
                title: 'Toplam Soru Sayısı: '
            };
            var chart = new google.visualization.PieChart(document.getElementById('piechart'));
            chart.draw(data, options);
        }

//call getDatas Function to receive JSON result

        function getDatas(){
            //call web service to get json
            WebService.getDatas(function (data) {
                if (data) {
                   //put data in scope.datas
                    $scope.datas =data;
                    drawChart();
                }
            });
        }

Also HTML code:

<div data-ng-controller="AnalizController">
    <div class="panel panel-primary">
        <div class="panel-heading">
            <h3>Analiz</h3>
        </div>
        <div class="panel-body">

           <div class="AnalyzeWrapper">
               <div class="AllQuestions">
            <div id="piechart" style="width: 40%; height: 40%"></div>
               </div>
           </div>
        </div>
    </div>
</div>

But I got an error in HTML page:

Table has no columns.