Javascript JSON记录计数返回0？ExtJS4

这是firebug中的记录返回，我正在尝试使用

alert（MarkerStore.getCount（））对记录进行计数返回0？为什么？

我仔细检查了字段名，所有字段都是正确的。这是怎么回事

Ext.define('GoogleMarkerModel', {
        extend: 'Ext.data.Model',
        fields: ['ID','Locating','MainPower','Acc','PowerOff','Alarm','Speed','Direction','Latitude','Longitude','DateTime','MainID','IOState','OilState']
    });

    var MarkerStore = Ext.create('Ext.data.JsonStore', {
        model: 'GoogleMarkerModel',
        autoLoad: true,
        proxy: {
            type: 'ajax',
            url: 'get-googlemarker.php',
            baseParams: {  //here you can define params you want to be sent on each request from this store
                        mainid: 'value1'
                        },
            reader: {
                type: 'json',
            }

        }
    });

MarkerStore.load({
params: {  
        mainid: 1,
        }
})

更新
我已经添加了idproperty，但仍然无法工作
我不明白为什么要使用idproperty

看看下面的例子，你会发现一个简短的例子。您的代理读取器看起来应该类似，json也应该类似。使用root属性。
您何时要求计数？因为ajax调用是异步的，所以您可能在加载存储之前查询该值。
[{"ID":"1808","Locating":"1","MainPower":"0","Acc":"1","PowerOff":"1","Alarm":"128","Speed":"0","Direction":"293","Latitude":"5.391788482666016","Longitude":"100.29693603515625","DateTime":"2013-02-19 15:44:36","MainID":"1","IOState":"0","OilState":"0"}]

Ext.define('GoogleMarkerModel', {
    extend: 'Ext.data.Model',
    idProperty:'MainID',
    fields: ['ID','Locating','MainPower','Acc','PowerOff','Alarm','Speed','Direction','Latitude','Longitude','DateTime','MainID','IOState','OilState']
});

var MarkerStore = Ext.create('Ext.data.JsonStore', {
    model: 'GoogleMarkerModel',
    autoLoad: true,
    proxy: {
        type: 'ajax',
        url: 'get-googlemarker.php',
        baseParams: {  //here you can define params you want to be sent on each request from this store
                    mainid: 'value1'
                    },
        reader: {
            type: 'json',
            idProperty : 'MainID',
        }

    }
});