我正在看一个mongoose的例子（link），它使用了来自官方Next.js GitHub帐户的Next.js。这是他们的模型（我删除了评论）：

import mongoose from 'mongoose'

const PetSchema = new mongoose.Schema({
  name: {
    type: String,
    required: [true, 'Please provide a name for this pet.'],
    maxlength: [60, 'Name cannot be more than 60 characters'],
  },
  owner_name: {
    type: String,
    required: [true, "Please provide the pet owner's name"],
    maxlength: [60, "Owner's Name cannot be more than 60 characters"],
  },
  species: {
    type: String,
    required: [true, 'Please specify the species of your pet.'],
    maxlength: [40, 'Species specified cannot be more than 40 characters'],
  },
  age: {
    type: Number,
  },
  poddy_trained: {
    type: Boolean,
  },
  diet: {
    type: Array,
  },
  image_url: {
    required: [true, 'Please provide an image url for this pet.'],
    type: String,
  },
  likes: {
    type: Array,
  },
  dislikes: {
    type: Array,
  },
})

export default mongoose.models.Pet || mongoose.model('Pet', PetSchema)

为什么要导出mongoose.models.Pet || mongoose.model('Pet', PetSchema)？我们可以只导出export default mongoose.model('Pet', PetSchema)吗？