How to set an iframe src attribute from a variable in AngularJS

I’m trying to set the src attribute of an iframe from a variable and I can’t get it to work…

The markup:

<div class="col-xs-12" ng-controller="AppCtrl">

    <ul class="">
        <li ng-repeat="project in projects">
            <a ng-click="setProject(project.id)" href="">{{project.url}}</a>
        </li>
    </ul>

    <iframe  ng-src="{{trustSrc(currentProject.url)}}">
        Something wrong...
    </iframe>
</div>

controllers/app.js:

function AppCtrl ($scope) {

    $scope.projects = {

        1 : {
            "id" : 1,
            "name" : "Mela Sarkar",
            "url" : "http://blabla.com",
            "description" : "A professional portfolio site for McGill University professor Mela Sarkar."
        },

        2 : {
            "id" : 2,
            "name" : "Good Watching",
            "url" : "http://goodwatching.com",
            "description" : "Weekend experiment to help my mom decide what to watch."    
        }
    };

    $scope.setProject = function (id) {
        $scope.currentProject = $scope.projects[id];
        console.log( $scope.currentProject );

    }
}

With this code, nothing gets inserted into the iframe’s src attribute. It’s just blank.

Update 1:
I injected the $sce dependancy into the AppCtrl and $sce.trustUrl() now works without throwing errors. However it returns TrustedValueHolderType which I’m not sure how to use to insert an actual URL. The same type is returned whether I use $sce.trustUrl() inside the interpolation braces in the attribute src="https://stackoverflow.com/questions/20045150/{{trustUrl(currentProjectUrl))}}" or if I do it inside the controller when setting the value of currentProjectUrl. I even tried it with both.

Update 2:
I figured out how to return the url from the trustedUrlHolder using .toString() but when I do that, it throws the security warning when I try to pass it into the src attribute.

Update 3:
It works if I use trustAsResourceUrl() in the controller and pass that to a variable used inside the ng-src attribute:

$scope.setProject = function (id) {
    $scope.currentProject = $scope.projects[id];
    $scope.currentProjectUrl = $sce.trustAsResourceUrl($scope.currentProject.url);
    console.log( $scope.currentProject );
    console.log( $scope.currentProjectUrl );

}

My problem seems to be solved by this, although I’m not quite sure why.