[{_id:1,comments:[{owner:"aaa",feedback:{userText:"nice",thumb:"up"}},{owner:"aab",feedback:{userText:"not nice",thumb:"down"}}]},{_id:2,comments:[{owner:"aac",feedback:{userText:"nice",thumb:"up"}}]},{_id:3,comments:[{owner:"aad",feedback:{userText:"not nice",thumb:"down"}},{owner:"aaa",feedback:{userText:"nice",thumb:"up"}}]}]

db.collection.aggregate([{"$match":{"comments.owner":"aaa"}},{"$unwind":"$comments"},{"$match":{"comments.owner":"aaa"}},{"$replaceRoot":{"newRoot":"$comments"}}])