text: const mongoose = require('mongoose')
  const User = require('./User')
  const Course = require('./Course')
  const schema = new mongoose.Schema({
    course: { type: mongoose.SchemaTypes.ObjectId, ref: 'Course'},
    user: { type: mongoose.SchemaTypes.ObjectId, ref: 'User'},
    content: { type: String },
    reply: { type: String },
  },{
    timestamps: true
  })
  
  module.exports = mongoose.model('Message', schema)