无法在课程mongoose中设置对用户文档的引用

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  name: String,
  // 其他用户字段...
});

const courseSchema = new mongoose.Schema({
  name: String,
  user: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User'
  }
});

// 定义虚拟属性
courseSchema.virtual('userRef', {
  ref: 'User',
  localField: 'user',
  foreignField: '_id',
  justOne: true
});

const User = mongoose.model('User', userSchema);
const Course = mongoose.model('Course', courseSchema);

// 使用虚拟属性进行查询
Course.findOne({ name: '课程名称' })
  .populate('userRef')
  .exec((err, course) => {
    if (err) {
      console.error(err);
    } else {
      console.log(course.userRef);
    }
  });