[{"_id":"123","dirty_to_clean":["5ddc36f5e9c5cd64b437b466","5ddc14b4e9c5cd64b437b2be",["5dd6d8d6e9c5cd64b437a127","5dd6a9bce9c5cd64b4379ef3","5dd5a594e9c5cd64b43799a9"]]}]

db.collection.aggregate([{$unwind:"$dirty_to_clean",},{$unwind:"$dirty_to_clean",},{$group:{_id:"$_id","dirty_to_clean":{$push:"$dirty_to_clean"}},},/**{*$out: {*db: "<output-db>",*coll: "<output-collection>"*}*}*/])