How to get two way data binding in a directive *without* an isolate scope?

Question

Using scope: { ... } in a directive introduces an isolate scope, which does not prototypically inherit from its parent scope. But I have always used it for a different reason: a convenient way to declare HTML attributes with two way data binding:

scope: {
    attr1: '=',
    attr2: '?='
}

To get a non-isolate scope, you have to use scope: true, which does not offer the opportunity to declare such attributes. I now find myself needing a directive with a non-isolate scope, but with two way binding. What's the best way to achieve this?

---

Example: My use-case is something like this, in the view of the outer-directive:

<div ng-repeat="e in element">
    <inner-directive two-way-attr="e.value"></inner-directive>
</div>

But inner-directive is in the same module as outer-directive. It doesn't need to be encapsulated with an isolate scope. In fact, I need to use $scope inheritance for other purposes, so an isolate scope is not an option. It's just that using an HTML attribute to establish this two-way communication is extremely convenient.

Answer 1

The answer by pixelbits helped me figure this out big time, but taken as a direct answer to my original question, it seems overly complicated. After looking into it, the solution is really quite simple.

Take a directive with an isolate scope like this:

scope: { model: '=myModel' },
link: function(scope, element, attr) {
    //...
}

The following is equivalent, except that the scope is not isolate:

scope: true,
link: function(scope, element, attr) {
    scope.model = scope.$parent.$eval(attr.myModel);
    //...
}

See a working example here: http://jsfiddle.net/mhelvens/SZ55R/1/