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PO
primarykey
Id
1121915
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
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0
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CreationDate
2009-07-13T20:42:10.813
FavoriteCount
0
LastActivityDate
2009-07-19T13:00:16.693
LastEditDate
2009-07-19T13:00:16.693
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0
OwnerUserId
0
ParentId
1033934
PostTypeId
2
Score
2
ViewCount
0
LastEditorDisplayName
Aryeh Leib Taurog
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Body
I came up with something similar to the Memento pattern, but different enough that I think it bears posting. When a user starts an editing session, I duplicate the target object to a temporary object in the database. All subsequent editing operations affect the duplicate. Instead of saving the object state in a memento at each change, I store operation objects. When I apply an operation to an object, it returns the inverse operation, which I store. Saving operations is much cheaper for me than mementos, since the operations can be described with a few small data items, while the object being edited is much bigger. Also I apply the operations as I go and save the undos, so that the temporary in the db always corresponds to the version in the user's browser. I never have to replay a collection of changes; the temporary is always only one operation away from the next version. To implement "undo," I pop the last undo object off the stack (as it were--by retrieving the latest operation for the temporary object from the db) apply it to the temporary and return the transformed temporary. I could also push the resultant operation onto a redo stack if I cared to implement redo. To implement "save changes," i.e. commit, I de-activate and time-stamp the original object and activate the temporary in it's place. To implement "cancel," i.e. rollback, I do nothing! I could delete the temporary, of course, because there's no way for the user to retrieve it once the editing session is over, but I like to keep the canceled edit sessions so I can run stats on them before clearing them out with a cron job.
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1. POPer-session transactions in Django
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