[{_id:ObjectId("6386ef039775398be3620c76"),age:34,"groups":[{"id":"d278c44333","name":"group 1"}],},{_id:ObjectId("6386ef239775398be3620c77"),age:24,"groups":[{"id":"d278c443335","name":"group 2"}],}]

db.collection.aggregate([{$project:{groupName:{$first:"$groups.name"}}}])