Re: Mixed PKs?
Re: Mixed PKs?
- Subject: Re: Mixed PKs?
- From: Giorgio Valoti <email@hidden>
- Date: Thu, 3 Jul 2003 00:11:31 -0700
On Mercoledl, lug 2, 2003, at 16:07 US/Pacific, Ricardo Strausz wrote:
On miircoles, juli 2, 2003, at 07:41 America/Mexico_City, Giorgio
Valoti wrote:
On Lunedl, giu 30, 2003, at 15:26 US/Pacific, B. Shank wrote:
I assume you're referring to compound keys. If so, see
http://developer.apple.com/documentation/WebObjects/
Enterprise_Objects/Saving/chapter_8_section_8.html#//apple_ref/doc/
uid/DontLinkBookID_460-DontLinkChapterID_8-BADJEBIB
Brent
On Monday, June 30, 2003, at 2:16PM, Ricardo Strausz wrote:
Hola!
does EOF supports multiple PKs generation?
Let me be more specific: in a one-to-many relation (say
Invoices->>Details) I want to give Details a double PK; the
invoices PK and a consecutive number.
Can EOF (in its lateest version) do this for me?
Gracias,
EOF cannot generate compound primary keys, nor a RDBMS can for that
matter. Compound PK are usually composed by two or more foreign keys,
so they cannot be automatically generated. However, an auto-generated
primary key can be used for performance reasons, but some more
constraints should be enforced.
In your specific case, the Details PK is enough to uniquely identify
a record: adding the Invoice PK brings no advantages.
In fact, it does; this brings back an old discussion on the uses of
PKs. There are people who stiks in the paradigm of using "non-sense"
PKs, but in the database I am modeling, the PKs have meaning to the
users (invoice number, and item in the example below) so the object is
unique up to these constrains (e.g., the third item in the invoice
number 1234).
Well, a PK can or cannot have a meaning to the user, I agree. My
position on this issue stems from the fact that when you a have
meaningful PK, you have to be prepared to deal with the possibility
that a PK could change, precisely because the PK has a "business
meaning". That's not necessarily bad, but it can be annoying, so I
prefer to avoid this, if I can. Obviously using a synthetic PK in your
case, leads to the another issue, namely that you have to enforce a
constraint about the uniqueness of the combination of the Invoice PK +
the number of the Item and that's not easy with EOF.
HTH
--
Giorgio Valoti
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