Custom input directive with ngModel

gistfile1.txt

angular
  .module('app', [])
  .directive('myThings', function() {
    return {
      require: 'ngModel',
      controllerAs: 'vm',
      bindToController: true,
      scope: {
        options: '=myThings'
      },
      template: '<p ng-repeat="option in vm.options"><label><input type="checkbox" name="selected[]" ng-model="isSelected" ng-init="isSelected = vm.isSelected(option)" ng-change="vm.toggle(option, isSelected)"> Option {{option.id}}</label></p><code>directive.model{{vm.model}} {{vm.isValid}}</code>',
      controller: function() {
        this.model = [];
        this.isSelected = function(option) {
          var index = this.model.indexOf(option);
          return index !== -1;
        }

        this.toggle = function(option, isSelected) {
          var index = this.model.indexOf(option);
          if (index === -1 && isSelected) {
            this.model.push(option);
          } else if (!isSelected) {
            this.model.splice(index, 1);
          }
        }
      },
      link: function(scope, el, attrs, ngModel) {
        ngModel.$validators.required = function(modelValue, viewValue) {
          var value;
          if (attrs.required) {
            value = modelValue || viewValue;
            return !!value.length;
          } else {
            return true;
          }
        };

        scope.$watchCollection('vm.model', function(value) {
          ngModel.$setViewValue(value);
          ngModel.$validate();
        });

        ngModel.$render = function() {
          if (!ngModel.$viewValue) {
            ngModel.$viewValue = [];
          }
          scope.vm.model = ngModel.$viewValue;
        }
      }
    };
  });