On Feb 20, 2007, at 9:17 AM, Ian Harding wrote:
On 2/17/07, Rick Schumeyer <rschumeyer@xxxxxxxx> wrote:
I have three tables of interest...Account, Employee, and
AccountEmployeeRelation. There is a many-to-many relationship
between
accounts and employees. The join table also contains a column
indicating what role the employee plays on this account.
My interface is a web app (I'm trying out Ruby on Rails). On the
"edit
account" screen I want to edit account attributes AND be able to
add/delete employees in one form. The gui part seems to work.
Be careful about some of the things Rails tries to push you towards,
such as "RI belongs in the application", and "surrogate key fields
should be named 'id'" (I *much* prefer the form "object_id", ie:
user_id, used *everywhere*, including the user table (in that
example)). Fortunately, with rails extensibility it shouldn't be hard
to change those default behaviors (in fact there's probably a patch
somewhere for the first case...)
BUT, when I update I'm not sure how to handle updating the
AccountEmployeeRelation table. During the update, relations may have
been added or deleted, and existing relations may have been
changed. It
seems to me the easiest thing to do is delete all the relations
for the
account and create all new ones with the data submitted from the
form.
This seems wasteful, but the alternative would be a pain. Or is this
really the best way?
I tried a bunch of cleverness where I checked for existence and
updated if required, etc but came back to just zapping them all and
inserting. As long as it's done in a transaction and there are not
too many, it's fine. It doesn't eat any more space and eats less
cycles than doing it the hard way.
Actually, zapping and re-creating everything *does* take more space,
due to how MVCC works in PostgreSQL. But so long as you're not doing
that a heck of a lot, it's probably not worth worrying about. It
might be worth detecting the case where nothing changes, though
(which I suspect could be done with creative use of INTERSECT and
it's ilk).
Thanks for any advice.
You're welcome!
Completely off topic, (but not worth a separate post) I have been
forced
to use a little bit of mysql lately...did you know that if you use
transaction and foreign key syntax with myisam tables, it does not
complain...it just silently ignores your requests for transactions
and
foreign key checks. Yikes! I had incorrectly assumed I would get an
error message indicating that transactions are not supported. Oh
well.
Sorry about that. Nuff said 8^/
That's one gotcha out of about 100. Google 'mysql gotchas' and hit
the first link.
--
Jim Nasby jim@xxxxxxxxx
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)
