[{"_id":"5e7229c248797243544a15b6","itinerary":[{"_id":"5e96f905c694d020bc99c2e4"},{"_id":"5e7230360bab9640f82a0e79"}]}]

db.collection.aggregate([{$project:{data:{$concatArrays:[[{_id:"$_id"}],{$map:{input:"$itinerary",in:"$$this"}}]}}},{$unwind:"$data"},{$replaceRoot:{newRoot:"$data"}}])