text: 'use strict'
  // external dependencies
  const mongoose = require('mongoose');   // mongoose; 'Model' in MVC
  const Schema = mongoose.Schema;
  const moment = require('moment');       // Moment.js; parsing, manipulation, and display of dates and times
  
  // Model which saves notification events
  const NotificationSchema = new Schema({
      location: { type: Schema.Types.ObjectId, ref: 'Post' },
      source: { type: Schema.Types.ObjectId, ref: 'User' },
      recipient: { type: Schema.Types.ObjectId, ref: 'User' },
      time: { type: 'Moment', default: new moment().unix() },
      type: { type: String },
      seen: { type: Boolean, default: false }
  })
  
  module.exports = mongoose.model('Notification', NotificationSchema);
  