node.js - 这是express下用mongolass创建一个数据库模型的代码，最后一句代码如何理解？

Question

exports.Post = mongolass.model('Post', {
author: { type: Mongolass.Types.ObjectId },
title: { type: 'string' },
content: { type: 'string' },
pv: { type: 'number' }
});
exports.Post.index({ author: 1, _id: -1 }).exec();// 按创建时间降序查看用户的文章列表 这里如何理解？

Answer 1

exports.Post.index({ author: 1, _id: -1 }).exec();

The above code is creating a composite index (author ascending order, _id descending order)

For reference.

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