Creating an isolate scope is undesirable. I would avoid using the scope attribute and do something like this. scope:true gives you a new child scope but not isolate. Then use parse to point a local scope variable to the same object the user has supplied to the ngModel attribute.
app.directive('myDir', ['$parse', function ($parse) {
return {
restrict: 'EA',
scope: true,
link: function (scope, elem, attrs) {
if(!attrs.ngModel) {return;}
var model = $parse(attrs.ngModel);
scope.model = model(scope);
}
};
}]);