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Altering data in GQL

Answer 1

+2 A:

logList = db.GqlQuery("SELECT * FROM Log ORDER BY date DESC LIMIT 1")
result = logList.get()
result.content = "some text"
result.put()

Try this. You are confusing the GqlQuery object for the results of actually eexecuting the query.

Adam Crossland 2010-09-27 19:02:15

Actually, you can access the results using the iterator interface. Moreover, when the "LIMIT" clause is specified, then "results are retrieved as with the equivalent `fetch()` method." (ref: [GqlQuery docs](http://code.google.com/appengine/docs/python/datastore/gqlqueryclass.html#Introduction))

David Underhill 2010-09-27 19:09:07

@David, he's not using the iterator interface; he is directly accessing an element of a suspected array. I don't know about the LIMIT=fetch() bit, but if I were him, I'd try what I suggested, as I think that his code looks a bit suspect to me. If he tries it and it doesn't work, I'll offer a bounty.

Adam Crossland 2010-09-27 19:14:41

Your suggestion definitely works (+1) :). I guess the documentation means that the results are retrieved in the same way, but not returned in the way (i.e., the results are still returned as an iterable). Looks like the page he referenced in the docs needs to be updated - it definitely seems to suggest that his method should work. I suspect the `db.put(logList)` in his code is just re-retrieving the entities, and thus the put is just saving a freshly fetched, un-changed copy.

David Underhill 2010-09-27 19:21:32

Adam, it worked! thanks :)

skazhy 2010-09-27 19:22:50

Every time you treat a Query like an iterator or an array, the query is executed afresh. Results are not cached between accesses, so the logList entry he fetches and modifies on line 2 is not the same as the set of entities that are stored back to the datastore (unmodified!) on line 3.

Nick Johnson 2010-09-28 13:56:45

@Nick: is that in the docs? While I never use the query-as-iterator/array pattern myself, I'll wager that many do, and it's likely causing some hidden performance problems.

Adam Crossland 2010-09-28 14:10:15

Yes, it's documented. Per http://code.google.com/appengine/docs/python/datastore/queryclass.html: "As with fetch(), the iterator interface does not cache results, so creating a new iterator from the Query object will re-execute the query." I also blogged about it in my 'gotchas' post: http://blog.notdot.net/2009/11/Python-Gotchas

Nick Johnson 2010-09-28 15:21:27

Yup, there it is right there. Cool, Nick, thanks.

Adam Crossland 2010-09-28 15:33:17

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Altering data in GQL

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