ng-model not updating when value changes

Question

Currently I have two date inputs. I am trying to make the second date input default to the first date input. The user can then change the second date input to be a different value (leaving the first unchanged).

The second date input is updating fine when I change my first date, however, the ng-model isn't modelling the value change to the form_data.to_date attribute and so this is resulting in the form still being invalid even though both required fields have been filled out.

Here is an example of the issue:

angular.module('myApp', [])
  .controller('myController', function($scope) {
    $scope.form_data = {
      from_date: '',
      to_date: ''
    };
    
    $scope.f = function() {
      setTimeout(function() { // delay to show that to_date is not updating
        console.log($scope.form_data);
      }, 1000);
    }
  });

angular.element(document).ready(() => {angular.bootstrap(document, ['myApp']);});

<script src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.7.5/angular.min.js"></script>
<div ng-controller="myController">
  
  <form name="myForm">
    <input type="date" 
           ng-model="form_data.from_date"
           name="from_date"
           ng-change="f()"
           required />
           
    <input type="date" 
           ng-model="form_data.to_date" 
           name="to_date"
           ng-value="form_data.to_date || form_data.from_date | date: 'yyyy-MM-dd'"
           required />
           
    <p>Form invalid: {{myForm.$invalid}}</p> <!-- Should be false when first date is changed -->
  </form>
</div>

How can I get the ng-model on the second date input to model its changes when the value is changed in the first input?

Answer 1

This is because you are using ng-value="form_data.to_date || form_data.from_date | date: 'yyyy-MM-dd'" i.e. if form_data.to_date exists then assign it, if not then assign form_data.from_date but it won't update the value of ng-model.

To do this you can modify your code like this:

    angular.module('myApp', [])
     .controller('myController', function ($scope) {
        $scope.form_data = {
          from_date: '',
          to_date: ''
        };
        $scope.f = function () {
          setTimeout(function () { // delay to show that to_date is not updating
            if ($scope.form_data.to_date == "") {
              $scope.form_data.to_date = $scope.form_data.from_date;
            }
            console.log($scope.form_data);
            $scope.$apply();
          })
        }
    });
    angular.element(document).ready(() => { angular.bootstrap(document, ['myApp']); });   

<script src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.7.5/angular.min.js"></script>
<div ng-controller="myController">
    <form name="myForm">
        <input type="date" ng-model="form_data.from_date" name="from_date" ng-change="f()" required />
        <input type="date" ng-model="form_data.to_date" name="to_date"
            ng-value="form_data.from_date | date: 'yyyy-MM-dd'" required />
        <p>Form invalid: {{myForm.$invalid}}</p> <!-- Should be false when first date is changed -->
    </form>
</div>