db={"users":[{_id:ObjectId("604caef28aa89e769585f0c4"),baseData:{firstName:"max",username:"max_muster"}}],"activitiesFeed":[{_id:ObjectId("604cb97c99bbd77b54047370"),userID:ObjectId("604caef28aa89e769585f0c4"),activity:ObjectId("604caf718aa89e769585f0c8"),viewed:false}],"activities":[{_id:ObjectId("604caf718aa89e769585f0c8"),"baseData":{"userID":ObjectId("604caef28aa89e769585f0c4"),"type":0,"title":"Alessandro send you a friend request.","description":"","actionID":ObjectId("604caef28aa89e769585f0c4")},}]}

db.activitiesFeed.aggregate([{$match:{userID:ObjectId("604caef28aa89e769585f0c4")},},{$lookup:{from:"activities",localField:"activity",foreignField:"_id",as:"lookup_activities",},},{$unwind:"$lookup_activities"},{$lookup:{from:"users",localField:"lookup_activities.baseData.actionID",foreignField:"_id",as:"lookup_users",},},])