[{person:[{"name":"Alex"},{"name":"Betsy"},{"name":"Chauncy"}]}]

db.collection.aggregate([{$unwind:{path:"$person",includeArrayIndex:"index"}},{$group:{_id:null,person:{$push:{k:{"$concat":["name",{$toString:"$index"}]},v:"$person.name"}}}},{$project:{person:{"$arrayToObject":"$person"}}},{"$replaceRoot":{"newRoot":"$person"}}])