The ORM Foundation

 Hi Terry,

I've noticed that once I add derivation rules, the derived fact types are erased from the relational view. Could you help me understand the reason for this?

For instance, for the attached diagram only one table is generated, the WBT, with two columns (WBTId and FTPCode).

Cheers 

 I suppose the underlying question would be:  What are the derivation rules?  If these truly are derived facts, i.e. derived from other information in the model, then that single-table map would be correct.That is the only fact not marked as derived! If you want them to be 'derived and stored', mark the derivation rule property for the fact type to 'derived and stored' - you'll see a double star on the diagram, and it should also affect the relational mapping

Also, be wary of any fact type that uses the word 'is' for its predicate. If it really is an 'is' relationship, perhaps you want a subtype, or even a single object to represent both. 

 I have attached what I believe may be a better way to represent what your diagram appears to be attempting to represent, using subtypes. It carries these assumptions:

 All Content types either are WBT or are NOT WBT.

 All WBT Content types are stored in exactly one FTP

 An FTP may store any number of WBT content types.

[Edit] This is still technically incorrect, as there is (in the model) no way to determine if a given Content type is or is not a WBT. To correctly use subtypes, you need to have a subtype rule. In this instance, a simple unary fact on Content type, 'is WBT', and some text for the subtype rule would be enough.

thanks!

it is better. should have thought of it myself...

Hi Orion

Thanks for answering Rolemo's question. As a minor refinement to your answer, ORM 2 allows subtypes to be (simply) asserted, derived, or semiderived. So it is OK to simply assert the subtypes you show without a definition, if that is what you prefer. We hope soon to distinguish these 3 kinds of subtypes graphically as discussed in the BBB (append "*" to the name of derived subtypes, and append "+" to the name of semiderived subtypes).

Cheers

Terry

 Thanks!

That is good to know, Terry!

Hi Rolemo,

I hope that you are finding the website and feedback from other members to be useful.

However, I have been wondering if you realise that the answers to most of your questions are in Terry's 942 page "Big Brown Book". You could save yourself a lot of time by finding answers to your questions in the book and then discussing the things that you don't understand on the website.

The book contains lots of detail and lots of exercises and I can thoroughly recommend it.

Cheers
Ken