[{"_id":"post1","comments":[{"user":"goodman","text":"hello","replies":[{"user":"littleboy","text":"hi"}]},{"user":"richpapa","text":"haha"},{"user":"goodman","text":"how is it going?!"}]},{"_id":"post2","comments":[{"user":"kitten","text":"meow!","replies":[{"user":"kitten","text":"miaw miaw!"},{"user":"littleboy","text":"mioooooo"},{"user":"kitten","text":"meme"}]},{"user":"kitten","text":"ich bin katze","replies":[{"user":"sally","text":"ja, du bist katze maw"}]}]}]

db.collection.aggregate([{$unwind:"$comments"},{$project:{_id:0,items:{$filter:{input:"$comments.replies",as:"item",cond:{$eq:["$$item.user","$comments.user"]}}}}},{$unwind:"$items"},{$project:{"self_repling_user":"$items.user","self_repling_comment":"$items.text"}}])