text: const mongoose = require('mongoose')
  
  const userSchema = mongoose.Schema({
      // public address
      _id: {
          type: String,
          require: true,
          lowercase: true, 
          trim: true
      },
      // onetime random variable for signning purpose
      nonce: {
          type: Number,
          require: true,
          default: () => Math.floor(Math.random() * 1000000)
      },
      // edditable user name 
      username: {
          type: String,
          require: false
      }
  })
  
  module.exports = mongoose.model('User', userSchema)