Two way data binding with directive doesn't work - angularjs

Question

I have a controller used to add tasks. On that page a user needs to select a group to act upon. I have written a directive that is used to allow a user to pick groups (folders)
My page controller
function AddTaskController($scope) {
var vm = this;
vm.group = { whatsit: true };
$scope.$watch("vm.group", function () {
console.log("controller watch", vm.group);
},true);
}
The page html where the directive is used
<em-group-selection group="vm.group"></em-group-selection>
The directive configuration
function GroupSelectionDirective() {
return {
scope: {
group: '='
},
controller: GroupSelectionDirectiveController,
controllerAs: 'vm',
templateUrl: '/views/templates/common/folderselection.html'
};
}
The directive controller:
function GroupSelectionDirectiveController($scope) {
var vm = this;
$scope.$watch("group", function () { console.log("yo1", vm.group); }, true)
$scope.$watch("vm.group", function () { console.log("yo2", vm.group); }, true)
}
Now when this fires, both console.log() calls in the directive fire once, with undefined. They never fire again. If in the controller I set vm.group to something else the $watch in the AddTaskController never gets fired.
Why isnt the data binding working?
Update:
I notice that if, in the directive, I change the init() function in my directive to use $scope it works! Can I not, as Fedaykin suggests, use controllerAs with two way data binding?
function init() {
$timeout(function () {
$scope.group.shizzy = 'timeout hit';
}, 200);
}

Answer 1

Turns out that if you use isolate scopes and controlelrAs syntax you need to also use bindToController : true. Without this you will not be able to only use vm and will have to use $scope for the isolate scope variables
More information can be found in the John Papa style guide and this SO answer
The final directive setup is as so:
function GroupSelectionDirective() {
return {
scope: {
group: '='
},
controller: GroupSelectionDirectiveController,
controllerAs: 'vm',
bindToController: true,
templateUrl: '/views/templates/common/folderselection.html'
};
}