On Mon, Nov 2, 2009 at 1:41 PM, Mark L. <mark.l...@gmail.com> wrote:
>
> As the subject states, I am using the model-validation branch at the
> moment, which allows me to do, well, validation in the models.
>
> I've however, noticed a very strange behaviour of the validation
> logic, - any time the clean() method is fired on the
> "materialized" (saved or retrieved from the database) model, that
> contains unique fields, the following query is fired (let me just give
> you an example):
>
> from django.db import models
>
> class A(models.Model):
> pass
>
> a = A.objects.create()
> a.clean()
>
> a.clean() results, among everything, in this: 'SELECT (1) AS "a" FROM
> "val_a" WHERE ("val_a"."id" = 1 AND NOT ("val_a"."id" = 1 ))'
>
> I might be missing something vital, but what is that "where" clause
> there for? What is it supposed to check?
It's trying to verify the uniqueness of the supposed-to-be-unique field id.
The query makes sense for fields other than the primary key, where the WHERE
clause turns out more like:
WHERE model.unique_field = proposed_value AND NOT model.id =
current_instance.pk
That is, it's asking "Is there any other instance in the DB, besides this
one I'm looking at, that has the proposed_value for this
supposed-to-be-unique field?" If the answer is no, then all is fine, the
proposed value doesn't violate the unique constraint. If the answer is yes,
then the validation check needs to fail because saving this model instance
to the DB would violate the unique constraint.
Of course, it doesn't make sense for the primary key field. For that field,
when you exclude the current instance from the search the WHERE clause
becomes impossible to satisfy. The model validation code, I'd guess, has
been moved from where it was initially implemented (ModelForms). In its
original place the primary key field would not ordinarily have been included
in the list of unique fields to check because it wouldn't ordinarily be on
the form, and only values on the form are checked for uniqueness.
The code in the new place likely needs to realize the primary key field
shouldn't be checked for uniqueness. You could check to see if this has
been reported in trac and if not open a ticket for it. I don't recall it
having been brought up by anyone else but I could have missed it.
Karen
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