On Apr 17, 4:06 am, Malcolm Tredinnick <[EMAIL PROTECTED]>
wrote:
> > My "big stick" solution is just to override the save() method of the
> > model, and to have it swap the start and end fields.
>
> A conceptual problem with this approach is that it's doing validation in
> the wrong place. Validation and saving are intentionally separated in
> Django. The idea is that if you have any validation failure, you can go
> back to the data supplier and get more information. Saving should
> *never* raise a validation problem. We cannot avoid errors entirely,
> because database integrity errors may be thrown (another process,
> completely unrelated to Django may have put data in the database that
> caused the problem before we saved), however, in most cases, saving is a
> "safe" operation.
Malcolm is right when he's talking about validation of user-entered
data. However, invariants are also used to catch programming errors;
it's certainly possible to have buggy business logic code which causes
invariant checks to fail. An alternative to overriding the save()
method is to hook the pre_save event and call a model-class invariant
method to check if, from the model point of view, the model instance
is valid. (Note that the model point of view can be stricter and also
more complex than database constraints can easily capture.) If the
invariant fails, an exception can be thrown, and caught in the view to
display an appropriate user-friendly message.
Just my 2 cents,
Vinay Sajip
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