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Re: Objects and Relations

From: David BL <davidbl_at_iinet.net.au>
Date: 14 Feb 2007 16:46:12 -0800
Message-ID: <1171500372.256081.58920@v45g2000cwv.googlegroups.com>


On Feb 15, 12:36 am, "Kevin Kirkpatrick" <kvnkrkpt..._at_gmail.com> wrote:
> On Feb 14, 7:12 am, "Cimode" <cim..._at_hotmail.com> wrote:
>
>
>
>
>
> > [Snipped crap from Keith Duggar]
>
> > > Nevertheless, for the benefit of other readers if nothing else, let me
> > > state that Keith has hit the nail on the head.
>
> > No. Keith has just once shown more misunderstanding of RM and
> > arrogance.
>
> > > 1) Lego blocks are /not/ the same. They always have a different
> > > location attribute.
>
> > BS. LegoBlock are indeed the same if they are to be a relation.
>
> > Stating that location attribute is a primary natural key for a
> > LegoBlock is arbitrary and does not reflect the reality of
> > LegoBlocks. Imagine a M:N cardinality between LegoBlock and Location
> > Entity. In that case LocationEntity has a natural identifier (X, Y,
> > Z). The natural identifier of a lego block could be a combination of
> > its shape, color but certainly not its location.
>
> > > 2) Hence their x,y,z position attribute always identifies them.
>
> > False.
>
> > > 3) However this identifer is very hard to record and keep track of,
> > > even though it exists.
>
> > In the mind of people which pointer's obsessed maybe...
> > There is nothing complex about the identifier of a location is a
> > concatenation of XYZ and a referential.
>
> > > 4) So we represent it with a surrogate identifer (which is hence just
> > > an 'untrackable' attribute or, as others refer to it, an unfamiliar
> > > attribute).
>
> > In other words, Surrogate identifier has only the advantage of being
> > *familiar*.
>
> > > 5) OID's are hence not needed, and everything is just a value, as it
> > > should be.
>
> > OID = Implementation of Surrogate Key IF AND ONLY IF
> > --> unicity contraint has been implemented on the implementation of
> > the natural key
> > --> unicity constraint implemented on the OID
> > --> contraint of cardinality implemented 1:1 between the
> > implementation of the natural key and the OID....
>
> > In other words, almost never happens...
>
> > > 6) When I finally understood this (and it was a hard mental slog to
> > > shake off my OO mindset) I found it quite revelationary to my
> > > perspective on data management.
>
> > Guess we all went through this at some point in time...
>
> > [rest snipped]
>
> I have to agree with Cimode - Keith's contribution seemed a little...
> off. Assuming lego blocks are of the simple cuboid variety with
> standard male and female connectors, a configuration can be expressed
> as tuples in the following relation (asterisks indicate the natural
> key columns):
>
> CONNECTION (male_color*, male_thickness*, male_num_cols*,
> male_num_rows*, male_column*, male_row*, female_color*,
> female_thickness*, female_num_cols*, female_num_rows*, female_column*,
> female_row* , rotational_angle)

How can this work? There is nothing to distinguish the individual Lego blocks. Your key for a connection only makes use of the /type/ of Lego block (parameterised by colour and shape).

>
> Of course, this is not a complete schema - depending on application
> needs, one might add other base relations to enforce entegrity and
> store other information; for instance:
> LEGO_BLOCK (color*, thickness*, num_cols*, num_rows*, price,
> inventory, ...)
> COLOR (color*)
>
> But the point is that there is NO need for "lego block entity"
> identifiers (which makes sense: an entity id might give the impression
> that instantiating a new red 2x3x3 LegoBlock object with OID #254 and
> swapping it out for an existing red 2x3x3 LegoBlock object with OID
> #120 somehow changes the configuration).- Hide quoted text -
>
> - Show quoted text -
Received on Wed Feb 14 2007 - 18:46:12 CST

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