[{"id":10,"mess":"root comment","postId":333,"parents":[10]},{"id":11,"mess":"reply to root","parents":[10,11],},{"id":12,"mess":"reply to reply to root","parents":[10,11,12],},{"id":13,"mess":"bottom most","parents":[10,11,12,13],}]

db.collection.aggregate([{"$match":{postId:333,parents:{"$size":1}}},{"$graphLookup":{"from":"collection","startWith":"$parents","connectFromField":"parents","connectToField":"parents","as":"replies",}},{"$addFields":{"replies":{"$filter":{"input":"$replies","as":"item","cond":{"$ne":[{"$size":"$$item.parents"},{"$size":"$parents"}]}}}}}])