JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html ng-app="app">
<head lang="en">
    <meta charset="UTF-8">
    <title></title>

    <script src="https://code.angularjs.org/1.4.0-beta.4/angular.js"></script>
    
    <script>
       
        angular.module('app', [])
               .controller('TestCtrl', function ($scope, $location) {
                    $scope.form = {};
                    
                    $scope.brands   = [{id: null, name: 'B1'}, {id: 1, name: 'B2'}, {id: 2, name: 'B3'}];
                    $scope.models   = [{id: 1, name: 'M1'}, {id: 2, name: 'M2'}];
                                        
                    $scope.$watchCollection('form', function(form){
                        
                        if ( ! form.brandId && form.modelId){
                            delete form.modelId;    
                        }
                        
                        console.log('load data ...');
                    });
                    
               });

    </script>

</head>

<body ng-controller="TestCtrl">
    Селекты:
    <select ng-model="form.brandId" ng-options="brand.id as brand.name for brand in brands"></select>
    <select ng-model="form.modelId" ng-options="model.id as model.name for model in models"></select>
</body>
</html>