update(fields, callback) {

    callback = callback || (() => {});

    this.constructor.query()
      .where({id: this.get('id')})
      .update(fields, (err, models) => callback(err, models && models[0]));

  }
Runs an update query for this specific model instance
@param {Object} fields Key-value pairs of fields to update
@param {Function} callback Callback to execute upon completion