Validation example for http://www.ng-newsletter.com/posts/validations.html

Validationexample.js

var app = angular.module('validationExample', []);

app.controller('signupController', ['$scope', function($scope) {
  $scope.submitted = false;
  $scope.signupForm = function() {
    if ($scope.signup_form.$valid) {

    } else {
      $scope.signup_form.submitted = true;
    }
  }
}]);

app.directive('ensureUnique', ['$http', '$timeout', function($http, $timeout) {
  var checking = null;
  return {
    require: 'ngModel',
    link: function(scope, ele, attrs, c) {
      scope.$watch(attrs.ngModel, function(newVal) {
        if (!checking) {
          checking = $timeout(function() {
            $http({
              method: 'POST',
              url: '/api/check/' + attrs.ensureUnique,
              data: {'field': attrs.ensureUnique}
            }).success(function(data, status, headers, cfg) {
              c.$setValidity('unique', data.isUnique);
              checking = null;
            }).error(function(data, status, headers, cfg) {
              checking = null;
            });
          }, 500);
        }
      });
    }
  }
}]);

Try to use attrs.ngModel instead of attrs.ensureUnique

if you swap out line 20:

if (!checking) {

with a check for field is $dirty

if (!checking && c.$dirty ) {

then you won't get an 'invalid' when the form loads, and is pristine.