AngularJS Validation on <select> with a prompt option

Answer 1

JS

    angular.module('interfaceApp')
  .directive('requiredSelect', function () {
    return {
      restrict: 'AE',
      require: 'ngModel',
      link: function(scope, elm, attr, ctrl) {

        if (!ctrl) return;
          attr.requiredSelect = true; // force truthy in case we are on non input element

          var validator = function(value) {
            if (attr.requiredSelect && ctrl.$isEmpty(value)) {
              ctrl.$setValidity('requiredSelect', false);
              return;
            } else {
              ctrl.$setValidity('requiredSelect', true);
              return value;
            }
          };

          ctrl.$formatters.push(validator);
          ctrl.$parsers.unshift(validator);

          attr.$observe('requiredSelect', function() {
            validator(ctrl.$viewValue);
          });
      }
    };
  });