text: const mongoose = require('mongoose')  //引用mongoose
  
  const schema =new mongoose.Schema({   //创建模型结构
      title: { type: String },
      categories: [{type: mongoose.SchemaTypes.ObjectId, ref: 'Category'}],
      body:{type:String}
  })
  
  module.exports = mongoose.model("Article", schema)  //创建模型并导出