We're seeing numerous oopses each day on reviews.ubuntu.com for posts to (for example) /reviews/api/1.0/reviews/11039/recommendations/
with tracebacks like: https://pastebin.canonical.com/49101/
We lose the original trackback because a second exception is recorded while piston-miniclient tries to log the error, but there's enough info to be certain that the issue is in reviewsapp.api.handlers.SubmitUsefulnessHandler.create:
{{{
usefulness, created = Usefulness.objects.get_or_create(
review=review, user=request.user)
}}}
The MultipleObjectsReturned seems to imply that it is already possible (somehow) for a user to create 2 recommendations for a single review and we're erroring when they try to create a third :). Checking the model, there is currently no db constraint for this on the Usefulness model... we should add one with a migration to delete duplicates while fixing the error.
The software center spec doesn't provide any reason for multiple recommendations against the same review. In fact, we built the client to keep track of usefulness submitted and hide the yes/no buttons if the user has already submitted usefulness for that review.
But it was implemented in Natty with only a local cache which is updated on submit of usefulness, which means it is not perfect. For Oneiric, I've already re-written it to retrieve the usefulness votes from the server for the currently logged in user so it should be more reliable.
In any event, the server should probably stop multiples from being possible.