Database
[{"_id":"410a7cb2-7ee1-4e7a-9fb7-fa651fcaa4e5","reqHistoryEvents":[{"reqHistoryMsg":"abcd","reqHistoryCreatedAt":ISODate("2022-04-27T08:18:30.850Z"),},{"reqHistoryMsg":"EFGH ","reqHistoryCreatedAt":ISODate("2022-04-27T08:22:12.716Z"),},{"reqHistoryMsg":"IJKL","reqHistoryCreatedAt":ISODate("2022-04-27T08:22:12.716Z"),}]}]
Query
db.collection.aggregate([{$unwind:"$reqHistoryEvents"},{$group:{_id:{_id:"$_id",reqHistoryCreatedAt:"$reqHistoryEvents.reqHistoryCreatedAt"},reqHistoryMsg:{$push:"$reqHistoryEvents.reqHistoryMsg"}}},{$group:{_id:"$_id._id",reqHistoryEvents:{$push:{reqHistoryMsg:"$reqHistoryMsg",reqHistoryCreatedAt:"$_id.reqHistoryCreatedAt"}}}}])