Mongo模式，具有唯一值的字符串数组

const express = require('express');const router = express.Router();const mongoose = require('mongoose');const _ = require('underscore');let sampleSchema = new mongoose.Schema({  name: {    type: 'String',    unique: true  },  tags: [{    type: 'String'  }]})sampleSchema.pre('save', function (next) {  this.tags = _.uniq(this.tags);  next();});const Sample = mongoose.model('sample', sampleSchema, 'samples');router.post('/sample', function (req, res, next) {  const sample = new Sample(req.body);  sample.save()    .then((sample) => {      return res.send(sample);    })    .catch(err => {      return res.status(500).send(err.message);    })});