Thanks Eli. Your solution solves one issue. But ideally, I want the 2nd concurrent request, after waiting, pick up the next item in queue and process, without complicating the code. After all, that's what a transaction is designed for.
Reading another post, it seems one has to create a dummy parent entity for those entities you want to query in a transaction. What I'm looking for is a guide with good examples for querying (otherwise) root entities in a transaction. Will On Sun, Nov 15, 2009 at 1:57 PM, Eli Jones <eli.jo...@gmail.com> wrote: > Nevermind, I see what you mean. > > So you want a lock on the entity once you know it exists (and satisfies > your where clause). > > ..since something might have come along and updated the p1 column from None > to Something before your transaction began.. and thus the key would refer to > an entity that does not satisfy your where clause anymore. > > If you were willing to let whoever got to it first to update the p1 > column.. you could have your transaction verify that p1 was still None once > it got the entity inside of the transaction. > > If p1 had changed.. then just do nothing since another transaction already > won the race. > > I think that would work just the same. Because, assume you have two > processes looking for an entity where p1 = None and p2 = Today.. and they > each really, really want to update p1 to some value. > > One of the processes must win in your scenario.. you just don't want the > other one to re-update the entity since it has already been updated.. and p1 > <> None. Now, you still need the transaction of course.. but just add a > check after grabbing the entity to verify that p1 = None before doing any > updates. > > > On Sun, Nov 15, 2009 at 12:20 AM, Eli Jones <eli.jo...@gmail.com> wrote: > >> you should read the section here: >> >> >> http://code.google.com/appengine/docs/python/datastore/functions.html#run_in_transaction >> >> where they show an example of iterating a counter in a transaction. >> >> If you don't already know the key for the object... then you sort of need >> to get it to run your transaction. In the decrement example, they grab the >> entity first so that they can send the entity key to the decrement function. >> >> Now, if you want to modify their decrement function to see if it works by >> getting the key inside of the transaction.. then just try that.. it's only >> a few lines of code. >> >> On Sat, Nov 14, 2009 at 10:29 PM, Will <vocalster....@gmail.com> wrote: >> >>> I want to find that particular entity, change it and save it back. When >>> one request is doing this, I don't want another request pick up the same >>> entity and modify it again. In short, I want the 'seek-change-save' to be >>> serialized, as one reason the GAE transaction is designed for. Finding out >>> the key of the entity first out of a transaction and then using the key >>> inside the transaction defeats the purpose. >>> >>> Will >>> >>> >>> On Sat, Nov 14, 2009 at 1:24 PM, 风笑雪 <kea...@gmail.com> wrote: >>> >>>> Why you need a transaction to update just one entity? Just save it and >>>> you may get an exception when update failed. >>>> >>>> BTW, DateTimeProperty has an parameter auto_now, you can use it to >>>> automatic update its time by datastore. >>>> >>>> And if you really want a transaction, you need fetch it before start >>>> the transaction: >>>> >>>> item = C1.gql("WHERE p1 = :a AND p2 < :b ORDER BY p2", a = None, b = >>>> today).get() >>>> if item: >>>> key = item.key() >>>> def update_time(key) >>>> item = C1.get(key) >>>> if item: >>>> item.p1 = now >>>> item.put() >>>> db.run_in_transaction(update_time, key) >>>> >>>> 2009/11/14 Will <vocalster....@gmail.com>: >>>> > Yes, I've read the document. I'm looking for some examples. >>>> > >>>> > As I said, I have a class, C1, whose entities have no ancestors. I >>>> want to >>>> > query a particular entity from it, modify, and put it back, for >>>> example, >>>> > >>>> > item = C1.gql("WHERE p1 = :a AND p2 < :b ORDER BY p2", a = None, b = >>>> > today).fetch(1) >>>> > item.p1 = now >>>> > item.put() >>>> > >>>> > Both p1 and p2 are datetime. How can I build this into a transaction? >>>> Is >>>> > there a way I can give the entities a 'place holder' ancestor so I can >>>> later >>>> > use an 'ancestor filter' in the query to satisfy the requirement? >>>> > >>>> > Thanks, >>>> > >>>> > Will >>>> > >>>> > >>>> > On Fri, Nov 13, 2009 at 9:44 PM, Stephen <sdea...@gmail.com> wrote: >>>> >> >>>> >> On Nov 13, 2:27 am, Will <vocalster....@gmail.com> wrote: >>>> >> > >>>> >> > Can you give me some examples of 'ancestor queries'? If possible, >>>> I'd >>>> >> > like >>>> >> > to change the existing ones into 'ancestor queries' and fit the >>>> whole >>>> >> > into a >>>> >> > transaction, because that is exactly what a transaction designed >>>> for. >>>> >> >>>> >> >>>> >> >>>> >> >>>> http://code.google.com/appengine/docs/python/datastore/transactions.html#What_Can_Be_Done_In_a_Transaction >>>> >> >>>> >> -- >>>> >> >>>> >> You received this message because you are subscribed to the Google >>>> Groups >>>> >> "Google App Engine" group. >>>> >> To post to this group, send email to >>>> google-appeng...@googlegroups.com. >>>> >> To unsubscribe from this group, send email to >>>> >> google-appengine+unsubscr...@googlegroups.com<google-appengine%2bunsubscr...@googlegroups.com> >>>> . >>>> >> For more options, visit this group at >>>> >> http://groups.google.com/group/google-appengine?hl=. >>>> >> >>>> >> >>>> > >>>> > -- >>>> > >>>> > You received this message because you are subscribed to the Google >>>> Groups >>>> > "Google App Engine" group. >>>> > To post to this group, send email to >>>> google-appeng...@googlegroups.com. >>>> > To unsubscribe from this group, send email to >>>> > google-appengine+unsubscr...@googlegroups.com<google-appengine%2bunsubscr...@googlegroups.com> >>>> . >>>> > For more options, visit this group at >>>> > http://groups.google.com/group/google-appengine?hl=. >>>> > >>>> >>>> -- >>>> >>>> You received this message because you are subscribed to the Google >>>> Groups "Google App Engine" group. >>>> To post to this group, send email to google-appeng...@googlegroups.com. >>>> To unsubscribe from this group, send email to >>>> google-appengine+unsubscr...@googlegroups.com<google-appengine%2bunsubscr...@googlegroups.com> >>>> . >>>> For more options, visit this group at >>>> http://groups.google.com/group/google-appengine?hl=. >>>> >>>> >>>> >>> -- >>> You received this message because you are subscribed to the Google Groups >>> "Google App Engine" group. >>> To post to this group, send email to google-appeng...@googlegroups.com. >>> To unsubscribe from this group, send email to >>> google-appengine+unsubscr...@googlegroups.com<google-appengine%2bunsubscr...@googlegroups.com> >>> . >>> For more options, visit this group at >>> http://groups.google.com/group/google-appengine?hl=. >>> >> >> > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine" group. > To post to this group, send email to google-appeng...@googlegroups.com. > To unsubscribe from this group, send email to > google-appengine+unsubscr...@googlegroups.com<google-appengine%2bunsubscr...@googlegroups.com> > . > For more options, visit this group at > http://groups.google.com/group/google-appengine?hl=. > -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. 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