nurl/models/url.js

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const CounterSchema = Schema({
  _id: { type: String, required: true },
  seq: { type: Number, default: 1000 }
});

const counter = mongoose.model('counter', CounterSchema);

// create a schema for our links
const urlSchema = new Schema({
  _id: { type: Number, index: true },
  long_url: String,
  created_at: Date,
  visits: { type: Number, default: 0 }
});

urlSchema.pre('save', function(next) {
  const doc = this;
  counter.findByIdAndUpdate({ _id: 'url_count' }, { $inc: { seq: 1 } }, function(error, counter) {
    if (error)
      return next(error);
    doc.created_at = new Date();
    doc._id = counter.seq;
    next();
  });
});

const Url = mongoose.model('Url', urlSchema);

module.exports = Url;