Backbone Collection.sort导致模型属性变为空

Question

在Backbone集合上调用sort方法会导致title属性在所有成员模型上都设置为空。请注意，代码是用Typescript编写的。

模型定义

export class Field extends Backbone.Model{ //Form field backbone model 
    id: number;
    event_id: number;
    title: string;
    mandatory: boolean;
    type: number;
    type_data: Array<any>;
    required: boolean;
    temp_option: string;
    sort_order: number;
    defaults(){
        return {
            "id":null,
            "event_id": null,
            "title": "",
            "mandatory": false,
            "type": 1,
            "type_data": [],
            "required": true,
            "temp_option": "",
            "sort_order": 0
        };
    }
};

收藏品定义

export class FieldCollection extends Backbone.Collection<Models.Field>{
    url = function() {
        return tsExports.apiPath + '/form/' + this.id;
    };
    id : number;
    model = Models.Field;
    initialize(models : Array<Models.Field>, options : any) {
        this.id = options.id;
    };
    comparator = function(item : Models.Field) { //to allow the sort function to work properly
        return item.get('sort_order');
    };
    unserialize(){
        this.each(function(item : Models.Field, key : Number){
            item.set('type_data', JSON.parse(item.get('type_data')));
        });
    }
}

排序代码：

var a = Fields.get(fieldId); //fields is the collection. Essentially swaps positions with the adjacent model.
var b = Fields.findWhere({sort_order: a.get('sort_order')-1});
b.set('sort_order', b.get('sort_order')+1);
a.set('sort_order', a.get('sort_order')-1);
Event.fields.sort(); 

排序前后：