Vadim Macagon:
> ...
> Let's say I have a Post model with a title field, that title must be
> unique for a given date. Here's a sequence of events that could
> potentially happen:
>
> time: t1). User A fills in a form to create a new Post, and sets the
> title to "Donkeys".
> time: t2). User B fills in a form to create a new Post, and sets the
> title to "Donkeys".
> time: t3). User A submits the form, server validates it to ensure title
> is unique and discovers nothing wrong.
> time: t3). User B submits the form, server validates it to ensure title
> is unique and discovers nothing wrong.
> time: t4). Server tries to commit user A's new Post to the db, commit is
> successful.
> time: t5). Server tries to commit user B's new Post to the db, commit
> fails and throws an IntegrityError.
> ...
That's a good spot.
It's hard to impossible to catch the error and figure out what the
error message means, since there's no real standard how these error
messages look like. All you got is a string ...
One way to do this is to lock the table Post before validation
starts (and unlock with the end of the transaction). There's no real
support for this in the Django ORM (or is there? I'm not 100 %
sure), but you can do it with custom SQL.
The other would be to catch the database error and then validate
again. But in the meantime, something else could have happened, so
you end up with a structure like this:
for i in range(settings.RETRIES)
... validate ...
try:
... save ...
break
except ...
... roll back ...
You don't want to loop forever, since it could be an error that is
not related to validation, like that the database is unavailable.
Both approaches don't look very appealing if you need to do this for
every form. The generic views don't (I think). Perhaps there should
be some support for this. If we can find a way to deal with this in
a generic way, that could be added to Django.
Michael
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