Custom form validation directive to compare two fields

Question

I'm an angular newbie, and I'm stumbling over something in how angular's form validation directives work.

I know that I can fairly easily add directives to individual fields, but I'm trying to add a validation which will compare two form fields (both of which are elements of a model).

Here's a form skeleton:

<form name="edit_form" >
  <input name="min" type="number" ng-model="field.min"/>
  <input name="max" type="number" ng-model="field.max"/>
</form>

<div class="error" ng-show="edit_form.min.$dirty || edit_form.max.$dirty">
  <small class="error" ng-show="(what goes here?)">
    Min cannot exceed max
  </small>
</div>

In short, I want to write a directive and use it to show/hide this small.error if min and max both have values but min > max. How can I access both fields inside one directive? Is a directive the right tool for this job?

Answer 1

You do not need any directive. Just assign the "min" value of max to min-value. Like:

<input name="min" type="number" ng-model="field.min"/>
<input name="max" type="number" ng-model="field.max" min=" {{ field.min }}"/>

And you do not need any customization.
More: you can do min=" {{ field.min + 1}}"

Answer 2

Many ways to skin a cat.

PLUNKER

app.directive('lowerThan', [
  function() {

    var link = function($scope, $element, $attrs, ctrl) {

      var validate = function(viewValue) {
        var comparisonModel = $attrs.lowerThan;

        if(!viewValue || !comparisonModel){
          // It's valid because we have nothing to compare against
          ctrl.$setValidity('lowerThan', true);
        }

        // It's valid if model is lower than the model we're comparing against
        ctrl.$setValidity('lowerThan', parseInt(viewValue, 10) < parseInt(comparisonModel, 10) );
        return viewValue;
      };

      ctrl.$parsers.unshift(validate);
      ctrl.$formatters.push(validate);

      $attrs.$observe('lowerThan', function(comparisonModel){
        // Whenever the comparison model changes we'll re-validate
        return validate(ctrl.$viewValue);
      });

    };

    return {
      require: 'ngModel',
      link: link
    };

  }
]);

Usage:

<input name="min" type="number" ng-model="field.min" lower-than="{{field.max}}" />
<span class="error" ng-show="form.min.$error.lowerThan">
  Min cannot exceed max.
</span>

Answer 3

Would a simple comparison suit you?

<small class="error" ng-show="field.min > field.max">

I think a directive would be an overkill if your case is just this. If you do not feel comfortable with the view containing application logic, you can export it in a function of the controller:

$scope.isMinMaxInalid = function() {
    return $scope.field.min > $scope.field.max;
};

And the template:

<small class="error" ng-show="isMinMaxInalid()">

Answer 4

For me, beyond a feedback message, I needed define the field as invalid, preventing submit. So I gathered some approaches, like @thestewie approach, with a view configuration to gather a solution for dates comparison. I hope can aggregate the solutions that were presented.

The code is in PLUNKER

angular.module('MyApp')
    .directive('thisEarlierThan', function () {
        return {
            require: 'ngModel',
            restrict: 'A',
            link: function (scope, elem, attrs, ctrl) {
                var startDate,
                    endDate;

                scope.$watch(attrs.ngModel, function (newVal, oldVal, scope) {
                    startDate = newVal;
                    check();
                });

                scope.$watch(attrs.thisEarlierThan, function (newVal, oldVal, scope) {
                    endDate = newVal;
                    check();
                });

                var check = function () {
                    if (typeof startDate === 'undefined' || typeof endDate === 'undefined') {
                        return;
                    }

                    if (!validate(startDate)) {
                        startDate = new Date(startDate);
                        if (!validate(startDate)) {
                            return;
                        }
                    }

                    if (!validate(endDate)) {
                        endDate = new Date(endDate);
                        if (!validate(endDate)) {
                            return;
                        }
                    }

                    if (startDate < endDate) {
                        ctrl.$setValidity('thisEarlierThan', true);
                    }
                    else {
                        ctrl.$setValidity('thisEarlierThan', false);
                    }

                    return;
                };

                var validate = function (date) {
                    if (Object.prototype.toString.call(date) === '[object Date]') {
                        if (isNaN(date.getTime())) {
                            return false;
                        }
                        else {
                            return true;
                        }
                    }
                    else {
                      return false;
                    }
                };
            }
        };
    })
;

Answer 5

My version of the directive:

module.directive('greaterThan', function () {
    return {
        restrict: 'A',
        require: 'ngModel',
        link: function (scope, element, attributes, ngModelController) {
            var otherValue;

            scope.$watch(attributes.greaterThan, function (value) {
                otherValue = value;

                ngModelController.$validate();
            });

            ngModelController.$parsers.unshift(function (viewValue) {
                ngModelController.$setValidity('greaterThan', !viewValue || !otherValue || viewValue > otherValue);

                return viewValue;
            });
        }
    };
});

Answer 6

You may take a look at https://github.com/nelsonomuto/angular-ui-form-validation

This provides a directive that is preconfigured with an api that exposes the scope and its models to your validator function.

Here is a plunker with the your specific use case: http://plnkr.co/edit/S0rBlS?p=preview

The syntax for the directives validators is as shown in the below example : { errorMessage: 'Cannot contain the number one', validator: function (errorMessageElement, val, attr, element, model, modelCtrl){ /** * The model and modelCtrl(scope) are exposed in the validator function * */ return /1/.test(val) !== true;
} }