watch changes on JSON object properties - json

Question

I'm trying to implement a directive for typing money values.
var myApp = angular.module('myApp', []);
var ctrl = function($scope) {
$scope.amount = '0.00';
$scope.values = {
amount: 0.00
};
};
myApp.directive('currency', function($filter) {
return {
restrict: "A",
require: "ngModel",
scope: {
separator: "=",
fractionSize: "=",
ngModel: "="
},
link: function(scope, element, attrs) {
if (typeof attrs.separator === 'undefined' ||
attrs.separator === 'point') {
scope.separator = ".";
} else {
scope.separator = ",";
};
if (typeof attrs.fractionSize === 'undefined') {
scope.fractionSize = "2";
};
scope[attrs.ngModel] = "0" + scope.separator;
for(var i = 0; i < scope.fractionSize; i++) {
scope[attrs.ngModel] += "0";
};
scope.$watch(attrs.ngModel, function(newValue, oldValue) {
if (newValue === oldValue) {
return;
};
var pattern = /^\s*(\-|\+)?(\d*[\.,])$/;
if (pattern.test(newValue)) {
scope[attrs.ngModel] += "00";
return;
};
}, true);
}
};
});
HTML template:
<div ng-app="myApp">
<div ng-controller="ctrl">
{{amount}}<br>
<input type="text" style="text-align: right;" ng-model="amount" currency separator="point" fraction-size="2"></input>
</div>
</div>
I want to bind the value in my input element to values.amount item in controller but the watch instruction of my directive doesn't work.
How do I leverage two-way-data-binding to watch JSON objects?
To understand problem more precise I've created a jsfiddle.
The task is the following: Add extra zeros to the input element if user put a point. I mean if the value in input element say "42" and user put there a point, so the value now is "42." two extra zeros have to be aded like this "42.00".
My problems:
If I use ng-model="amount" the logic in input element works, but amount value of outer controller doesn't update.
If I use ng-model="values.amount" for binding, neither amount of outer controller nor input element logic works.
I really have to use ng-model="values.amount" instruction, but it doesn't work and I don't know why.
Any ideas?