Checkbox list breaks, why? And optimal Angularjs way? - angularjs

Question

I don't understand this, but I suspect I'm doing something wrong, or a non-angularjs way.
I have a checkbox list inside ng-repeat. It controller loads the list from a JSON. Pretty straightforward really. I'm then using a directive (car-select) on each of the resulting checkboxes. This directive calls a function inside the main $scope (selectBrand()). This cycles through the selected checkboxes, and if checked==true, add to $scope.brand. I've added a textbox so that $scope.brand fills it, and i've set it to required so that it fires the built in validation e.g:
HTML:
<div ng-repeat="v in viewModel">
<label class="checkbox">
<input type="checkbox" ng-model="v.c" ng-checked="v.c" />{{v.n}}
</label>
</div>
<input type="text" name="brands" ng-model="brands" car-select required/> <br>
JS:
$scope.selectBrand = function() {
var selectedBrands = [];
angular.forEach($scope.viewModel, function(v){
if (v.c)
selectedBrands.push(v.v);
})
if (selectedBrands.length > 0)
$scope.brands = selectedBrands;
else
$scope.brands = null;
}
DIRECTIVE:
app.directive('carSelect', function() {
return function(scope, element) {
element.bind('change', function() {
scope.selectBrand();
})
}
});
Here's the weird part which I don't understand. It took a while to figure out that this particular line was making this whole thing work. If I add the following in the page, everything works great. But if i remove it, the whole thing breaks. WHY?!
<div>{{selectBrand()}}</div>
It's like the whole thing doesn't bind unless the above is called in the HTML. It's called in the directive, and I've tried putting that call inside the clickButton() function, but eventually it breaks. Either way, the live update of the textbox seems to fail if the above is removed. I'd love to get a good explanation of how I'm doing something wrong and how I could fix it :)
PLUNKER:
http://plnkr.co/edit/4QISKcq7YYH678YLsTTF?p=preview

Answer 1

Ok, i create fork ;-)
update variable with only data checked
your model :
{"cars":
[
{"v":"m","n":"Mini","c":false},
{"v":"c","n":"Corvette","c":true},
{"v":"b","n":"BMW","c":true},
{"v":"l","n":"Lamborghini","c":true},
{"v":"f","n":"Ferrari","c":false}
]
}
you want only checked :
$scope.brands = $filter('filter')($scope.viewModel, {c: true});
when model change you want to update your variable so use watch in controller
$scope.$watch('viewModel', function(newval, oldval){
if (oldval != newval)
{
$scope.brands = $filter('filter')($scope.viewModel, {c: true});
}
},true
);
});
see http://plnkr.co/edit/PnABre?p=preview