text: var mongoose = require('mongoose');
  var Schema = mongoose.Schema;
  
  var CounterSchema = Schema({
      "_id": {type: String, required: true},
      "seq": { type: Number, default: 0 }
  });
  
  var counter = mongoose.model('counter', CounterSchema);
  
  // create a schema for our links
  var urlSchema = new Schema({
    "_id": {type: Number, index: true},
    "long_url": String,
    "created_at": Date
  });
  
  urlSchema.pre('save', function(next){
    var 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();
    });
  });
  
  var Url = mongoose.model('Url', urlSchema);
  
  module.exports = Url;
  