I created a nested category model. In order to select a category for a product, you see a dropdown with the main categories. When you select one, a new dropdown is added to the right with the child categories to the one you selected. This repeats until you reach the last level. When this happens $scope.product.category_id is set. I want to invalidate the whole set of fields when $scope.product.category_id is null. 
I've been using angular-bootstrap-show-errors for validation and I tried to mix it with ui-utils to achieve this one, using a custom function: validate_category().
Here's the HTML I used:
<span ng-repeat="parent_id in category_dropdown_parents" show-errors="{ skipFormGroupCheck: true }">
    <select class="form-control" name="category_id"
        ng-init="selected_category[$index] = init_select($index);"
        ng-model="selected_category[$index]"
        ng-options="category.name for category in (categories | filter : { parent_id: parent_id } : true ) track by category.id "
        ng-change="select_category(selected_category[$index], $index)"
        ui-validate="'validate_category()'" // added this to work with ui-utils
        >
    </select> 
    <span ng-if="$index+1 != category_dropdown_parents.length">/</span>
</span>
And this is my simple validation function:
$scope.validate_category = function() {
    return  (   $scope.product.category_id !== null 
            &&  $scope.product.category_id !== undefined);
}
But this is not working. Ideas?
EDIT: I just realized, that the problem with this is that the validation function on ui-validate is executed after the ng-change function, so it's never able to check the $scope.product.category_id update.
 
     
    