MW: As I explained, this does not work with a data model where you want to be able to extend your ontology in data, in particular being able to add subtypes as data instead of changing the data model by adding entity types each time you identify something new. Changing data models is expensive compared to changing data. You therefore need the entity type that the subtypes of e.g. activity would be instances of, and this is class_of_activity (or activity_type whatever your taste is).
So, perhaps defining class_of_activity as a special kind of class *is* peculiar, but useful, as a way to get run-time subclasses in a relational database.
MW: I don’t see why it should be considered peculiar. There is nothing strange or peculiar about powersets. The only question is whether they are useful in a particular context.
Perhaps I was misunderstanding your models as intended to be optimal for understand and formalizing the ontology of a domain of human endeavor,
while instead they are intended to optimally support that understanding in a way that can be used in a given kind of technology. A way to avoid the 'impedence mismatch' between classes or E/R entity types and relational tables.
MW: You are essentially correct, but I would put it slightly differently. For me there is an underlying ontology that includes the subtypes both directly and within the powerset, and depending on the implementation technology different parts come to the fore. For example, there is an OWL version of ISO 15926 that just uses the subtypes. Different implementation technologies have different limitations that you have to live within.
As below:
MW: Well I wish you luck when you are working for a client and saying every time they want a new category they have to change the data model rather than adding a data record.
This is indeed a problem, I can see how this might be a very good solution. I think there is more than one solution to it.
MW: There usually is, but the growing emphasis on Master and Reference Data Management, which generally follows this approach, suggests widespread adoption.
Regards
Matthew West
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