Re: Enforcing uniqueness of data
Re: Enforcing uniqueness of data
- Subject: Re: Enforcing uniqueness of data
- From: Arturo Pérez <email@hidden>
- Date: Wed, 7 Sep 2005 20:47:25 -0400
Chuck Hill wrote:
On Sep 7, 2005, at 10:00 AM, Daniel Eggert wrote:
I need the login of my user EO to be unique. Since the login is
something the user enters, it is not the PK.
How would I go about ensuring this? If I fetch from the database and
then write if it is unique, someone might be able to write to the
database in the meantime.
This post
<http://lists.apple.com/archives/webobjects-dev/2004/Jun/
msg00249.html>
suggests locking. Is it a sane approach? How would I do such a thing
with EOF?
I'd consider the other option that it mentions, adding a unique
constraint. You will then have to catch exceptions from save changes
and determine if the exception was a DB server side exception
(EOGeneralAdaptorException) and, if so, if the exception was raised
due to a violation of the unique constraint. Unfortunately, the
exceptions and errors returns from DB servers are vendor specific so
you will end up writing handling code for each server you need to
support.
You can do the locking too, and it is possible, but I can't whip the
EOF code off the top of my head.
Chuck
In my code I trap the EOGeneralAdaptorException, extract the
EOAdaptionOperation (which had failed), extract the JDBCException and
then check the SQL-92 standard SQLState provided by that. For a SQL
standard database the SQLState will contain a String of the form 23XXX
where the 23 denotes an integrity constraint failure (uniqueness,
check, etc). I think, for my database, the UNIQUE failure was a 23505
which, as it turns out, is standard SQL for a UNIQUEness failure.
So, in you situation, I do a saveChanges(), trap the exception as
above, if it's a 23505 I convert it into a validation error. I
personally did not like the idea of 2 hits against the database and the
possibility that the uniqueness of an ID could change while the user
was updating it.
YMMV,
-arturo
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