mongoose add virtual field or tojson

gistfile1.js

Schema.virtual('id').get(function(){
    return this._id.toHexString();
});

Schema.set('toJSON', {
    virtuals: true
});

// or

Schema.methods.toJSON = function(options) {
  var document = this.toObject(options);
  document.id = document._id.toHexString();
  delete(document._id);
  return document;
};