[{"id":"xxx","countryName":"xxx","cities":{"melbourne":{"id":"xxx","cityName":"xxx","population":124},"brisbane":{"column1":"xxx","column2":"xxx"}}}]

db.collection.aggregate([{"$project":{"cities":{"$objectToArray":"$cities"}}},{"$unwind":"$cities"},{"$replaceRoot":{"newRoot":{$arrayToObject:[[{"k":{"$concat":["cities",".","$cities.k"]},"v":"$cities.v"}]]}}}])