[{"_id":0,"a":1,"b":{"d":2},"c":[1,2,3]}]

db.collection.aggregate([{"$project":{"_id":0,"outer-fields":{"$map":{"input":{"$map":{"input":{"$objectToArray":"$$ROOT"},"in":["$$m.k","$$m.v"],"as":"m"}},"in":{"$arrayElemAt":["$$this",0]}}}}}])