无法读取未定义的属性“删除”

Question

我正在尝试在node和mongoose中创建一个登录页面，并且一旦我运行包含一些名为seed.js的测试数据的文件，就会弹出该错误

const mongoose = require('mongoose');

const db = require('./dbconn');

const users = [
  { username: 'username', password: 'password' },
  { username: 'kelvin', password: 'password' },
];

const seed = async () => {
  try {
    await db.User.remove();
    console.log('DROP ALL USERS');

    await Promise.all(
      users.map(async user => {
        const data = await db.User.create(user);
        await data.save();
      }),
    );
    console.log('CREATED USERS', JSON.stringify(users));

    await Promise.all(
      polls.map(async poll => {
        poll.options = poll.options.map(option => ({ option, votes: 0 }));
        const data = await db.Poll.create(poll);
        const user = await db.User.findOne({ username: 'username' });
        data.user = user;
        user.polls.push(data._id);
        await user.save();
        await data.save();
      }),
    );
    console.log('CREATED POLLS', JSON.stringify(polls));
  } catch (err) {
    console.error(err);
  }
};

seed();

还有我的schema.js文件，该文件位于名为dbconn-的文件夹中

const mongoose = require('mongoose');
const bcrypt = require('bcrypt');

var userSchema = new mongoose.Schema({
    username:{
        type:String,
        required:true,
        unique:true,
    }
    ,password:{
        type:String,
        required:true,
    },
    created:{
        type:Date,
        default:Date.now
    },
});
userSchema.pre('save',async function(next){
    try{
        if(!this.isModified('password')){
            return next();
        }
        const hashed = await bcrypt.hash(this.password,10);
        this.password=hashed;
        return next();
    }
    catch(err){
        return next(err);
    }

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

dbconn文件已经有一个index.js文件，该文件已经使用mongoose建立了mongodb连接