code: function (queryKey, params, fn) {
      this.connection.query(templates[queryKey], params, function (err, result) {
        if (err) {
          fn(err, null);
        } else {
          fn(null, result.length ? result[0] : null);
        }
      });
    }
docs: getOne('<sql template>', [constraint1, constraint2, ...], fn)