code: function setupAsSingle(scope, selectElement, ngModelCtrl, selectCtrl) {
          ngModelCtrl.$render = function() {
            var viewValue = ngModelCtrl.$viewValue;
  
            if (selectCtrl.hasOption(viewValue)) {
              if (unknownOption.parent()) unknownOption.remove();
              selectElement.val(viewValue);
              if (viewValue === '') emptyOption.prop('selected', true); // to make IE9 happy
            } else {
              if (isUndefined(viewValue) && emptyOption) {
                selectElement.val('');
              } else {
                selectCtrl.renderUnknownOption(viewValue);
              }
            }
          };
  
          selectElement.on('change', function() {
            scope.$apply(function() {
              if (unknownOption.parent()) unknownOption.remove();
              ngModelCtrl.$setViewValue(selectElement.val());
            });
          });
        }
docs: //////////////////////////