[{"a":{"ISODate":"2021-03-14T17:14:58.064Z"},"b":"EXECUTE IMMEDIATE"},{"a":{"ISODate":"2021-03-14T17:14:58.064Z"},"b":"EXECUTE IMMEDIATE"},{"a":{"ISODate":"2021-03-14T17:14:58.064Z"},"b":"EXECUTE IMMEDIATE"}]

db.collection.aggregate([{$replaceRoot:{newRoot:{"$arrayToObject":[[{k:"$a.ISODate",v:"$b"}]]}}}])