I created a gadget which is able to post activities:

function postActivity(text) {  
    var params = {};  
    params[opensocial.Activity.Field.TITLE] = text;
    var activity = opensocial.newActivity(params); 
    opensocial.requestCreateActivity(activity, opensocial.CreateActivityPriority.HIGH);
};

And I request activities by doing:

req.add(req.newFetchActivitiesRequest( new opensocial.IdSpec({'userId' : 'VIEWER', 'groupId' : 'FRIENDS'})), 'friends_activities');

My question is: How can I filter activities to only show activities posted by a specific gadget?

I reckon the field "appId" is meant for this, but this field is empty in the response.