// callback function - if error return it to callback. If no error then return rows (empty set)
  static insert (city, callback) {
    db.query('INSERT INTO cities (city_name) VALUES ($1)', [city], (err, res) => {
      if (err.error)
        return callback(err);
      callback(res);
    });
  }

// fetch an object that has a city property
  // after getting city list clear input field
  handleAddCity = () => {
    fetch('/api/cities', {
      method: 'post',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ city: this.state.newCityName })
    })
    .then(res => res.json())
    .then(res => {
      this.getCityList();
      this.setState({ newCityName: '' });
    });
  };