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From: "Simon St.Laurent" <simonstl@simonstl.com>
> >2) although the world is constantly changing, relationships need not
> >so constantly change. For example...the *relationship*
> ><#Simon> :sonOf _:1 between [you and your father] need not change
> >(I haven't even assigned a URI to your father!)
>
> If all relationships changed all the time, we'd all be pretty confused.
> The questions seem to arise around cases where changes in classification
> possible, necessary, and often unpredictable.
But relationships do change all the time. <#Simon> :sonOf_:1 is as good an
example as any. I assume the 1 is intended to be the cardinality, so the
relationship might be better named sonOfFather. But if your mother divorces
and remarries, sonOfFather may no longer be many-to-one, but many-to-many.
The latter probably better captures the relationship in the US, where
roughly half of first marriages end in divorce. Wasn't so in the early part
of the last century; it's a culturally induced change. Moreover, if you are
cloned, you may not have a father, at all; which is quite a different thing
than having a father but not knowing who he is. When this fairly recent
eventuality, the examples of Vulcan and Jesus Christ notwithstanding, comes
to pass, the cardinality of the father end of the relationship will become
0:n.
Now consider the "type" of the father, nominally a male Person. Suppose a
woman is impregnated by a sperm donor at a fertility clinic, where the
identity of the donor is kept secret. This could be modeled as an unknown
father, but in fact the identity of the biological father is known by the
clinic, and the name of the clinic can serve as a surrogate for the father.
If the type of the father is changed to the union of Person|Clinic, the
parentage information is not lost. More children than one might suppose
would benefit from this change.
The point about relationships being neither homogeneous nor stable over
time, in either number or type, nor universally applicable, has been well
made by William Kent, e.g., in his classic "Limitations of Record-Based
Information Models" ACM TODS paper. Many of Kent's examples are cases where
relationships change to meet business needs, e.g., cars can be leased to
departments as well as individual drivers, which are changes in business
practice.
I agree with the other points you made, but I hate to see you concede this
one.
Bob Foster
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