Changing a class with ng-class within a ng-repeat

I'm writing a small Ionic/Angular app and have run into an issue.

I have a list of events created with ng-repeat. Each event has a button which allows me to set a reminder for that event. I want to change the classes on this button when the lists loads (if a reminder if already set - this is working) and ALSO when you click the button to set/unset a reminder (this is not working).

I have 2 controllers in use here - the agenda controller that gets the data for the list and the notification controller that deals with setting the reminder when the button is clicked.

How can I set a variable per button that can be used both onload and when the button is pressed for ng-class to use?

Note - the notifications setting works fine, it's just the button class I can't work out how to change when it's clicked.

Heres the code so far.

My list / buttons:

<ion-list>
        <ion-item ng-repeat="(id,agendaItem) in agendaItems" 
          type="item-text-wrap" 
          href="#/tab/agendaItem/{{agendaItem.$id}}" 
          class="item item-avatar item-with-grid">
            <img ng-src="{{agendaItem.image}}">
            <p>{{agendaItem.startTime}}</p>
            <h2>{{agendaItem.title}}</h2>
            <p>{{agendaItem.speaker}}</p>
            <ion-option-button 
              ng-class="agendaItem.hasNotificationSet 
                          ? 'button-balanced icon ion-ios7-bell' 
                          : 'button-positive icon ion-ios7-bell-outline'" 
              ng-controller="NotificationCtrl" 
              ng-click="add(agendaItem.notificationId)"></ion-option-button>
        </ion-item>
</ion-list>

My agenda controller:

.controller('AgendaCtrl', function($scope, AgendaFactory, $ionicSideMenuDelegate, $rootScope, $ionicPopup, $timeout, $cordovaLocalNotification, NotificationFactory) {

    var agendaItems = AgendaFactory.getAgenda();

    // Loop through agenda itens and check which have notifications set
    agendaItems.$loaded().then(function(array) {
        angular.forEach(array, function(value, key) {
            console.log(value.notificationId);
            if (NotificationFactory.isNotificationScheduled(value.notificationId)) {
                console.log("scheduled");
                value.hasNotificationSet = true;
            } else {
                console.log("NOT scheduled");
                value.hasNotificationSet = false;
            }
        });

        $scope.agendaItems = agendaItems;
        console.log($scope.agendaItems);
    });
})

And the notifications controller:

.controller('NotificationCtrl', function($scope, $cordovaLocalNotification, NotificationFactory, $ionicListDelegate) {

$scope.add = function(notificationId, startTime) {

    console.log("NOTIFY: add("+notificationId+")");

    // Check if notification already set for this ID
    NotificationFactory.isNotificationScheduled(notificationId)
        .then(function(isScheduled) {
            console.log("returned data = " + isScheduled);

            // If notification is already scheduled, cancel it
            if(isScheduled) {

                // does this need a then and return on the factory????
                NotificationFactory.cancelNotification(notificationId).then(function(data) {
                    console.log(data);
                    console.log("cancelled from add() as preexisting")''
                    $ionicListDelegate.closeOptionButtons();
                });

            // Notification is not already scheduled 
            } else {
                // Add notification
                console.log("notification is not set, proceeding with creating a new one");

                var alarmTime = new Date();
                alarmTime.setMinutes(alarmTime.getMinutes() + 1); // needs to be the event time from firebase minus 5 mins

                NotificationFactory.addNotification(notificationId, alarmTime, "message", "title").then(function(added) {
                    if(added) {
                        console.log("The notification has been saved");
                        $ionicListDelegate.closeOptionButtons();
                    } else {
                        // always end up here?
                        console.log("ERROR saving notification");
                    }
                });
            }

        }, function(error) {
            console.log('error', error);
        }
    );
}
})