ListingSchema.js

/* Import mongoose and define any variables needed to create the schema */
var mongoose = require('mongoose'), 
    Schema = mongoose.Schema;

/* Create your schema for the data in the listings.json file that will define how data is saved in your database
     See https://mongoosejs.com/docs/guide.html for examples for creating schemas
     See also https://scotch.io/tutorials/using-mongoosejs-in-node-js-and-mongodb-applications
  */
var listingSchema = new Schema({
  /* Your code here */ 
  code: { type: String, required: true, unique: true },
  name: { type: String, required: true, unique: false },
  coordinates: {
    latitude: {type: Number, required: false, unique: false},
    longitude: {type: Number, required: false, unique: false}
  },
  address: {type: String, required: false, unique: false},
  created_at: Date,
  updated_at: Date
});

/* Create a 'pre' function that adds the updated_at (and created_at if not already there) property 
   See https://scotch.io/tutorials/using-mongoosejs-in-node-js-and-mongodb-applications
*/
listingSchema.pre('save', function(next) {
  /* your code here */
  var currentDate = new Date();

  this.updated_at = currentDate;

  if (!this.created_at)
    this.created_at = currentDate;

  next();
});

/* Use your schema to instantiate a Mongoose model */
var Listing = mongoose.model('Listing', listingSchema);

/* Export the model to make it avaiable to other parts of your Node application */
module.exports = Listing;