How to fix 'Can't insert key undefined, it violates the unique constraint' error in NeDB?

Question

I'm trying to use NeDB for my vue.js app, but the following error occurred when I tried to save my data from the second time.

The Error is:

"Can't insert key undefined, it violates the unique constraint."

I'm making the app in electron-vue.

I checked the database file and find that no _id field had been generated. I think the _id field should be generated automatically.

src\renderer\datastore.js

import Datastore from 'nedb'
import path from 'path'
import { remote } from 'electron'
export default new Datastore({
  autoload: true,
  filename: path.join(remote.app.getPath('userData'), '/data.db')
})

src\renderer\main.js

import db from './datastore'
Vue.prototype.$db = db

src\renderer\components\MyApp.vue

<template>
  <div>
    <form v-on:submit.prevent="insertShelf">

      <ul>
        <li>
          <label>Shelf Number</label>：
          <input type="text" name="shelfNum" v-model="newInput.shelfNum">
        </li>
      <ul v-for="(book, index) in newInput.books">
        <li>
          <label>Title</label>：
          <input type="text" name="title" v-model="book.title">
        </li>
        <li>
          <label>Author</label>：
          <input type="text" name="author" v-model="book.author">
        </li>
      </ul>
      <input type="submit" value="Submit">
    </form>
  </div>
</template>

<script>
export default {
  data () {
    return {
      newInput: {
        shelfNum: '',
        books: [
          {
            title: '',
            author: ''
          }
        ]
      },
      shelfList: []
    }
  },
  watch: {
    shelfList: {
      handler: function () {
        this.$db.insert(JSON.stringify(this.shelfList), function (err, newDoc) {
          console.log('err', err)
          console.log('newDoc', newDoc)
        })
      }
    }
  },
  methods: {
    insertShelf: function () {
      this.shelfList.push({
        shelfNum: this.newInput.shelfNum,
        books: this.newInput.books
      })

      this.newInput = {
        shelfNum: '',
        books: [
          {
            title: '',
            author: ''
          }
        ]
      }
    }
  }
}
</script>

At the first time, the data I filled the form and send is saved successfully, but from the second time the error occurred.

The database file ( C:\Users\myaccount\AppData\Roaming\Electron\data.db ) has only this:

"[{\"shelfNum\":\"12345\",\"books\":[{\"title\":\"Winnie-the-Poo\",\"author\":\"A. A. Milne\"}]}]"

and _id field dose not be generated.

Please help me.