使用 mongoose 时遇到的导入导出问题

// 引入 mongoose
const mongoose = require('../db/index')

//定义 sechema
const userSchema = mongoose.Schema(
    {
        username: {
            type: String,
            required: true,
            unique: true,
        },
        password: {
            type: String,
            required: true,
        },
        age: {
            type: Number,
            default: 0,
        },
        city: {
            type: String,
            default: "",
        },
        gender: {
            type: Number,
            default: 0, //默认保密
        }
    },
    {
        timestamps: true,
        versionKey: false,
    }
)

//定义模型
const userModel = mongoose.model('user',userSchema);

//输出模型
module.exports = userModel;