问在Mongoose架构中使用_id作为属性类型时出错

EN

var mongoose = require('mongoose');
var User = require('../users/user');

var notesSchema = new mongoose.Schema({
    author: User.userId,
    text: String,
    files:[String]
});

var archiveSchema = new mongoose.Schema({
    name: String,
    priority: String,
    deadline: Date,
    status: String,
    assigned_memnbers: [User.userId],
    notes: [notesSchema],
  });

  archiveSchema.virtual('archiveId').get(function() {
    return this._id;
});

module.exports = mongoose.model('Archive', archiveSchema);

var mongoose = require('mongoose');

var userSchema = new mongoose.Schema({
    username: String,
    mail: String,
    bio: String,
    password: String
});

userSchema.virtual('userId').get(function() {
    return this._id;
});

module.exports = mongoose.model('User', userSchema);

TypeError: Invalid value for schema path `author`, got value "undefined"