var User = db.define('user', {
    username: {type: Sequelize.STRING, allowNull: false, unique: 'compositeIndex'},
    password: {type: Sequelize.STRING, allowNull: false, },
    lastAt: {type: Sequelize.DATE, allowNull: false, defaultValue: Sequelize.NOW,},
    roleType: {
        type:   Sequelize.INTEGER,
        allowNull: false,
        defaultValue: Enumeration.roleType[3].value,

    },
},{
    indexes: [
        // Create a unique index on username,password
        {
            unique: true,
            fields: ['username','password']
        }
    ],
    getterMethods   : {
        roleTypeTitle : function() { return Enumeration.roleType[this.roleType].title },
        lastAtFormat : function() { return tools.formatDate(this.lastAt) },
        createdAtFormat: function() { return tools.formatDate(this.createdAt) },
        updatedAtFormat: function() { return tools.formatDate(this.updatedAt) },
    },
})

var User = db.define('user', {
    username: {type: Sequelize.STRING, allowNull: false, unique: 'compositeIndex'},
    password: {type: Sequelize.STRING, allowNull: false, },
    lastAt: {type: Sequelize.DATE, allowNull: false, defaultValue: Sequelize.NOW,},
    roleType: {
        type:   Sequelize.INTEGER,
        allowNull: false,
        defaultValue: Enumeration.roleType[3].value,

    },
},{
    indexes: [
        // Create a unique index on username,password
        {
            unique: true,
            fields: ['username','password']
        }
    ],
    getterMethods   : {
        roleTypeTitle : () => Enumeration.roleType[this.roleType].title,
        lastAtFormat :  () =>  tools.formatDate(this.lastAt) ,
        createdAtFormat:  () => tools.formatDate(this.createdAt) ,
        updatedAtFormat:  () => tools.formatDate(this.updatedAt) ,
    },
})

以上两段代码，折腾了一个晚上，第二段代码 使用es6的写法， 里面的this 对象 和 第一段代码中的this 对象不一样的。

请问，这里的getterMethods 如果用 es6的 写法，怎样去获取原来的那个this？