[Rails] Re: validates :uniqueness apparently doesn't

Fritz Anderson wrote in post #1108098:
> ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-darwin12.3.0]
> Rails 3.2.13
>
> I have a validates ... :uniqueness constraint on one of the attributes
> of an ActiveRecord class.
>
> In my test suite, I set the attribute from the same attribute in a
> record in the fixture. I then send invalid? to the object under test.
> invalid? returns _false_, and the .errors object for the record shows no
> errors.
>
> A :uniqueness constraint on another attribute does invalidate the
> record. All validations on other attributes behave as expected.
>
> I've done what I could to search the Rails docs, Google, Stack Overflow,
> and this forum for similar problems, and found nothing.
>
> What's wrong?

I didn't take the time to dig through your test code thoroughly, but
there are a few things you need to know about uniqueness validation on
ActiveRecord? The primary issue is that uniqueness is not guaranteed.
AFAIK there still exists a race condition that could cause duplicates to
be allowed in the database. This is especially true when there are
multiple instances of an app running. Or when records can be added
outside of the application.

Again, AFAIK the ONLY way to guarantee uniqueness is by using a unique
index on the column at the database level. Validation login in an ORM
(like ActiveRecord IS NOT sufficient). It is convenient to have because
it's easier to recover from a uniqueness violation, but it does not
remove the necessity of having the unique constraint in the database.

A basic explanation of this can be found here:
http://guides.rubyonrails.org/active_record_validations_callbacks.html#uniqueness

I have a strong suspicion that your issue is related to this limitation.

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