Database
[{Ticket:{"eventHistory":[{"event":"CREATED","timestamp":"aa-bb-cccc"},{"event":"ASSIGNED","timestamp":"ii-jj-kkkk"},{"event":"CLOSED","timestamp":"xx-yy-zzzz"}]}}]
Query
db.collection.update({},[{$set:{"Ticket.closedAt":{$last:"$Ticket.eventHistory.timestamp"}}}])