Angular directive for custom validation - angularjs

Question

I'm trying to create the AngularJS directive, which should vilidate the form. The form has two inputs. The valid case is when the imputs are either both empty or both filled with any string value.
<form name="recipientsForm" novalidate>
<md-input-container>
<label>Name</label>
<input name="name" type="text" ng-model="relationship.name">
</md-input-container>
<md-input-container>
<md-select name="type" placeholder="Select your relation... " ng-model="relationship.relationshipType">
<md-option ng-repeat="type in relationshipTypes" value="{{type.relationshipType}}">
{{type.name}}
</md-option>
</md-select>
</md-input-container>
</form>
The divective should be generic for other forms with such kind of validation.
My approach seems not working:
.html:
<form name="recipientsForm" novalidate>
<md-input-container>
<label>Name</label>
<input name="name" type="text" ng-model="relationship.name" value="" empty-or-both-filled="relationship.relationshipType">
<div ng-messages="recipientsForm.name.$error">
<div ng-message="emptyOrBothFilled">Enter name.</div>
</div>
</md-input-container>
<md-input-container>
<md-select name="type" placeholder="Select your relation... " ng-model="relationship.relationshipType" empty-or-both-filled="relationship.name">
<md-option ng-repeat="type in relationshipTypes" value="{{type.relationshipType}}">
{{type.name}}
</md-option>
</md-select>
<div ng-messages="recipientsForm.type.$error">
<div ng-message="emptyOrBothFilled">Pick relationship.</div>
</div>
</md-input-container>
</form>
.js file:
(function () {
'use strict';
angular
.module('app')
.directive('emptyOrBothFilled', [emptyOrBothFilled]);
function emptyOrBothFilled() {
return {
restrict: 'A',
require: 'ngModel',
scope: {
targetNgModel: '=emptyOrBothFilled'
},
link: function($scope, element, attrs, ngModel) {
ngModel.$validators.emptyOrBothFilled = function(val) {
console.log($scope.targetNgModel);
return (!val && !$scope.targetNgModel) || (!!val && !!$scope.targetNgModel);
}
$scope.$watch('targetNgModel', function() {
ngModel.$validate();
})
}
}
}
})();

Answer 1

An alternativ approach just using ng-disabled and ng-required:
<form name="recipientsForm" novalidate>
<md-input-container>
<label>Name</label>
<input name="name" type="text" ng-model="relationship.name">
<div ng-messages="recipientsForm.name.$error">
<div ng-message="emptyOrBothFilled">Enter name.</div>
</div>
</md-input-container>
<md-input-container>
<md-select name="type" placeholder="Select your relation... " ng-model="relationship.relationshipType" ng-disabled="relationship.name" ng-required="relationship.name">
<md-option ng-repeat="type in relationshipTypes" value="{{type.relationshipType}}">
{{type.name}}
</md-option>
</md-select>
</md-input-container>
</form>