#### JS ##########
angular.module('auth')
    .directive('uniqueEmail', function($http, $q) {
        return {
            restrict: 'A',
            require: 'ngModel',
            link: function(scope, elm, attrs, ctrl) {
                ctrl.$asyncValidators.uniqueEmail = function(modelValue, viewValue) {
                    return $http.post('/api/user/validate/email/', {email: modelValue}).then(
                        function(response) {
                            if (!response.data.isValid) {
                                return $q.reject(response.data.error);
                            }
                            return true;
                        },
                        function(response) {
                            return $q.reject("oops, something going wrong!");
                        }
                    );
                };
            };
        }
    });
##### HTML ##########
<form novalidate class="css-form" name="usrform">
    Email: <input type="email" ng-model="user.email" name="email" unique-email/><br/>
    <span ng-show="usrform.email.$error.uniqueEmail">addres is already in use</span>
</form>
<pre>email = "{{usrform.email}}"</pre>