Oops! [EMAIL PROTECTED] (Greg Copeland) was seen spray-painting on a wall: > --=-eu74lKXry3SVx8eZ/qBD > Content-Type: text/plain > Content-Transfer-Encoding: quoted-printable > On Fri, 2002-09-06 at 08:57, [EMAIL PROTECTED] wrote: >> > On Fri, 2002-09-06 at 07:37, Curt Sampson wrote: >> > > On 5 Sep 2002, Hannu Krosing wrote: >> > To elaborate on Gregs example if you have table GOODS and under it a >> > table CAMPAIGN_GOODS then you may place a general overridable constraint >> > valid_prices on GOODS which checks that you dont sell cheaper than you >> > bought, but you still want sell CAMPAIGN_GOODS under aquiring price, so >> > you override the constraint for CAMPAIGN_GOODS.
>> What that tells me is that the constraint, valid_prices, shouldn't >> have been on GOODS in the first place. If it is not a legitimate >> constraint for the children, then it is not a legitimate constraint >> for the parent. > I don't agree with you on that point. This concept is common to > many OO-implementations. Unless you can come up with a powerful > argument as to why our "to-be" picture should never do this, I'm > less than convinced. If the plan is for table CAMPAIGN_GOODS to virtually be a view on GOODS, then I'd say it _is_ necessary. >> In human inheritance, if you marry someone with "funny coloured skin," yo= > u=20 >> don't get to choose that your children won't have "funny coloured skin."= > =20=20 >> That's a pretty forcible "constraint." :-). >>=20 Is there something broken with your mailer? It's reformatting quotes rather horribly... > Fine, but that only works for YOUR specific example. In that > example, the color constraint should be non-virtual, meaning, the > child should not be able to change it. On the other hand, if I > replace "human" with "metal product", hopefully I won't be stuck > with gun metal gray for every derived product. Hopefully, somewhere > along the lines, I'll be able to override the parent's color > constraint. That happens by _adding_ an additional characteristic, presumably that of "what kind of paint the metal is covered with." That doesn't override the fundamental constraint that if it's a metal product, there _will_ be metallic properties. If you decide to add in some "non-metallic" products, then it would be _silly_ to have them inherit all their characteristics from "METAL_PRODUCTS;" they should head back up the class hierarchy and inherit their basic characteristics from the _appropriate_ parent. Reality, with the "GOODS/CAMPAIGN_GOODS" example, is that GOODS isn't the appropriate parent class for CAMPAIGN_GOODS. Both should be inheriting the common characteristics from some common ancestor. If that is done, then there's nothing to "override." >> > Or maybe it is better to just make the check function should be >> > dynamically dispatched, so the constraint will always hold, it just can >> > mean different things for different types. >>=20 >> Or maybe if someone is doing an Object Oriented design, and making extens= > ive=20 >> use of inheritance, they'll need to apply constraints in a manner that al= > low=20 >> them to be properly inherited. > The problem with that assumption is that there is normally nothing > wrong with having seemingly mutually exclusive sets of *business > rules* for a parent and child. If the rules are totally different, it begs the question of why they _should_ be considered to be related in a "parent/child" relationship. It may well be that they _aren't_ related as "parent/child." They may merely be "cousins," sharing some common ancestors. -- (concatenate 'string "chris" "@cbbrowne.com") http://cbbrowne.com/info/spreadsheets.html "Note that if I can get you to `su and say' something just by asking, you have a very serious security problem on your system and you should look into it." -- Paul Vixie, vixie-cron 3.0.1 installation notes ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])
