Mongoose深度populate关联字段时虚拟属性未返回问题求解

问题复现 #

深度populate关联的Comment模型虚拟属性formatted_date无法随接口响应返回，普通字段返回正常。
问题接口代码：

exports.get_user_feed = async (req, res, next) => {
  const options = { sort: { date: -1 } };
  const user = await User.find(
    { username: req.params.user },
    "username posts avatar followers following"
  )
    .populate({
      path: "posts",
      options,
      populate: [
        { path: "author" },
        { path: "comments", populate: { path: "author" } },
      ],
    })
    .sort({ "posts.date": 1 });
  res.json({ ...user });
};

Comment模型定义：

const mongoose = require("mongoose");
const { DateTime } = require("luxon");
const Schema = mongoose.Schema;

const CommentSchema = new Schema({
  targetPost: { type: Schema.Types.ObjectId, ref: "Post", required: true },
  author: { type: Schema.Types.ObjectId, ref: "User", required: true },
  date: { type: Date, required: true },
  content: { type: String, maxlength: 400 },
  comments: [{ type: Schema.Types.ObjectId, ref: "Comment" }],
  stars: [{ type: Schema.Types.ObjectId, ref: "User" }],
});

CommentSchema.virtual("url").get(function () {
  return "/" + this.targetPost.url + this._id;
});

CommentSchema.virtual("formatted_date").get(function () {
  return (
    DateTime.fromJSDate(this.date).toLocaleString(DateTime.DATE_MED) +
    " at " +
    DateTime.fromJSDate(this.date).toLocaleString(DateTime.TIME_SIMPLE)
  );
});

module.exports = mongoose.model("Comment", CommentSchema);

根本原因 #

Mongoose文档默认在序列化为JSON/普通对象时不会输出虚拟属性，和populate的嵌套深度没有关系，只要没有显式开启虚拟属性序列化配置，任何层级的虚拟属性都不会被返回。
另外原代码直接用展开运算符处理Mongoose文档数组，也会跳过Mongoose内部的序列化逻辑，增大属性丢失概率。

修复步骤 #

• 给CommentSchema添加配置，开启序列化时的虚拟属性输出：

const CommentSchema = new Schema({
  // 原有字段定义保持不变
  targetPost: { type: Schema.Types.ObjectId, ref: "Post", required: true },
  author: { type: Schema.Types.ObjectId, ref: "User", required: true },
  date: { type: Date, required: true },
  content: { type: String, maxlength: 400 },
  comments: [{ type: Schema.Types.ObjectId, ref: "Comment" }],
  stars: [{ type: Schema.Types.ObjectId, ref: "User" }],
}, {
  // 开启toJSON、toObject时的虚拟属性输出
  toJSON: { virtuals: true },
  toObject: { virtuals: true }
});

• 调整接口返回逻辑，避免直接展开Mongoose文档：
  因为User.find返回的是匹配用户的数组，如果是按唯一用户名查询，建议改用User.findOne，再调用toJSON()做序列化后返回：

exports.get_user_feed = async (req, res, next) => {
  const options = { sort: { date: -1 } };
  const user = await User.findOne(
    { username: req.params.user },
    "username posts avatar followers following"
  )
    .populate({
      path: "posts",
      options,
      populate: [
        { path: "author" },
        { path: "comments", populate: { path: "author" } },
      ],
    })
    .sort({ "posts.date": 1 });
  res.json(user.toJSON());
};

如果Post、User模型也有需要返回的虚拟属性，给对应Schema加上相同的toJSON/toObject配置即可。

内容的提问来源于stack exchange，提问作者filroy-codes