db={"notifications":[{_id:1,content:"some content 1 somegroup",targetGroup:"somegroup"},{_id:2,content:"some content 2 somegroup",targetGroup:"somegroup"},{_id:11,content:"some content 1 somegroup2",targetGroup:"somegroup2"}],"seenNotifications":[{_id:1,notificationId:1,userName:"johndoe"}]}

db.notifications.aggregate([{$match:{targetGroup:"somegroup"}},{$lookup:{from:"seenNotifications",localField:"_id",foreignField:"notificationId",as:"seenNotificationsDocs"}},{$match:{seenNotificationsDocs:{$size:0}}}])