The ORM Foundation

Hi again,

I'm trying to understand the difference between these two diagrams.

I understand that there's a verbalization difference:

• in the case of the derivation it means that the true value of one fact is derived from the other fact
  
• in the case of the subset it means that the set of completed projects is a subset of thesuperset Project completed on Date

 Still, the bottom line is that a Project must be completed on some date in roder to be completed. So when should I use each?

Thanks again (BTW, Terry - huge thanks for the labs, they're great ).

Rolemo

For this example, I would choose to make the fact type "Project is completed" as simply derived. The subset constraint approach would create an extra Boolean column "isCompleted" which would then have to be kept in sync with the completionDate column (isCompleted set to true/false if completionDate is not-null/null). It seems easier to simply derive this on update rather than store it.

Cheers

Terry

P.S. Ken is creating a NORMA Tutorials folder in the library that has the latest versions of the NORMA labs and related docs.

Sure good news about Ken's folder (though I hope i wont need it by the time its available). I may suggest to include a glossary. The only one I found (thanks to you actually) is focused on annotations, but hardly explains the proper use of them.

What I'm thinking of is more of a lexical or encyclopedic type of glossary, presenting each concept or symbol and explaining in a sentence or few:

• when and how to use it
• common mistakes
• what you must not forget when using it (an object must have a reference mode and data type)
• alternative annotations and when to choose them

I'd include some FOL basic concepts & relational DB as well.

Now i know preparing it would be a mighty task, but it will be an interesting one

 Cheers