Extjs5 - idProperty dosen't take effect

I have defined a custom Ext.Model, which is called itrip.model.City, And the primary key of this model is cityId, so I changed the default idProperty from 'id' to 'cityId', but it turns out a little bit werid, when I call Ext.model.load() to request my server, the actual request url turns out to be

http://localhost:8080/admin/city?_dc=1422349415598&id=itrip.model.City-1

see the param is still 'id' and the value is 'itrip.model.City-1', I don't know how is this come about. hope anyone could help :-)

heres my code:

Ext.define('itrip.model.City', {
        extend: 'Ext.data.Model',
        alias: 'city',
        idProperty: 'cityId',
        fields: [
                {name: 'cityId', type: 'int'},
                {name: 'countryId', type: 'int'},
                {name: 'cityNameZh', type: 'string'},
                {name: 'cityNameEn', type: 'string'},
                {name: 'cityCode', type: 'string'},
                {name: 'show', type: 'int'}
        ],
        proxy: {
            type: 'ajax',
            url: 'city',
        }
});

var city = itrip.model.City.load(1, {
        success: function(city1) {
      //omit...  
    }
);