auto-update directive that uses restangular

autoupdate.js

  .directive("autoUpdate", function(debounce_timeout){
    return {
      require: "ngModel",
      link: function(scope, element, attrs, ngModel){
        var model_path_array = attrs.ngModel.split(".");
        var field_name = _.last(model_path_array);
        var model_name = _.chain(model_path_array).last(2).first().value();


        function save_model_to_backend(){
          var model = scope[model_name];
          var patch_obj = _.object([field_name], [ngModel.$modelValue]);
          model.patch(patch_obj)
            .then(function(response){
              angular.extend(model, response);
            });
        }

        ngModel.$viewChangeListeners.push(_.debounce(save_model_to_backend, debounce_timeout));
      }
    };
  });

something.html

   <div
           class="form-group col-xs-4">
          <label class="sr-only"
                 for="{{delete_reason.id}}_title">
            DELETE TITLE
          </label>
          <input
             id="{{delete_reason.id}}_title"
             class="form-control"
             ng-model="delete_reason.title"
             auto-update
             type="text"
             placeholder="DELETE TITLE"
             required
             />
        </div>