text: var mongoose = require('mongoose');
  var Schema = mongoose.Schema;
  
  // 这种具名化的文件就是专门用来创建独立的model的
  
  
  var schema = new Schema({
      title: { type: String },
      categories: [{ type: mongoose.SchemaTypes.ObjectId, ref: 'Category' }], //因为一篇文章可能属于公告又可能属于热门，属于多个分类
      body: { type: String },
  }, {
      timestamps: true,
  });
  
  module.exports = mongoose.model('Article', schema)