[{"_id":{"$oid":"64e929386a2bff904d86118a"},"name":"Copies","legacy_id":181},{"_id":{"$oid":"64e929386a2bff904d86118b"},"name":"Temp","legacy_id":182},{"_id":{"$oid":"64e929386a2bff904d86118c"},"name":"Test","legacy_id":183}]

db.collection.aggregate([{"$group":{"_id":null,"records":{"$push":{k:{"$toString":"$_id"},v:"$legacy_id"}}}},{"$replaceRoot":{"newRoot":{"$arrayToObject":"$records"}}}])